refactor: cleaner header parsing

Builds off seanmonstar#134 (swar), seanmonstar#138 (Bytes cursor)

Cleaner, faster and less macros !

## Key changes

- Broke down header-parsing into clean conceptual steps (whilst being faster !)
- Added InnerResult allowing for idiomatic `?` early-exits, removing need for parsing-helper macros
- Removed macros.rs, leaving only `byte_map!` (response header-parsing macros should become functions)

### TODO

- convert request header-parser, supporting its quirks

src/iter.rs

@@ -1,108 +1,157 @@
-use core::slice;
-use core::convert::TryInto;
-use core::convert::TryFrom;
-
 #[allow(missing_docs)]
 pub struct Bytes<'a> {
-    slice: &'a [u8],
-    pos: usize
+    start: *const u8,
+    end: *const u8,
+    cursor: *const u8,
+    phantom: core::marker::PhantomData<&'a ()>,
 }
 
 #[allow(missing_docs)]
 impl<'a> Bytes<'a> {
     #[inline]
     pub fn new(slice: &'a [u8]) -> Bytes<'a> {
+        let start = slice.as_ptr();
+        let end = unsafe { start.add(slice.len()) };
+        let cursor = start;
         Bytes {
-            slice,
-            pos: 0
+            start,
+            end,
+            cursor,
+            phantom: core::marker::PhantomData,
         }
     }
 
     #[inline]
     pub fn pos(&self) -> usize {
-        self.pos
+        self.cursor as usize - self.start as usize
     }
 
     #[inline]
     pub fn peek(&self) -> Option<u8> {
-        self.peek_ahead(0)
+        if self.cursor < self.end {
+            // SAFETY: bounds checked
+            Some(unsafe { *self.cursor })
+        } else {
+            None
+        }
     }
 
     #[inline]
     pub fn peek_ahead(&self, n: usize) -> Option<u8> {
-        self.slice.get(self.pos + n).copied()
+        let ptr = unsafe { self.cursor.add(n) };
+        if ptr < self.end {
+            // SAFETY: bounds checked
+            Some(unsafe { *ptr })
+        } else {
+            None
+        }
     }
-
+    
     #[inline]
-    pub fn peek_n<U: TryFrom<&'a[u8]>>(&self, n: usize) -> Option<U> {
-        self.slice.get(self.pos..self.pos + n)?.try_into().ok()
+    pub fn peek_n<U>(&self) -> Option<U> {
+        let n = core::mem::size_of::<U>();
+        // Boundary check then read array from ptr
+        if self.len() >= n {
+            let ptr = self.cursor as *const U;
+            let x = unsafe { core::ptr::read_unaligned(ptr) };
+            Some(x)
+        } else {
+            None
+        }
     }
 
     #[inline]
     pub unsafe fn bump(&mut self) {
-        debug_assert!(self.pos < self.slice.len(), "overflow");
-        self.pos += 1;
+        self.advance(1)
     }
 
-    #[allow(unused)]
     #[inline]
     pub unsafe fn advance(&mut self, n: usize) {
-        debug_assert!(self.pos + n <= self.slice.len(), "overflow");
-        self.pos += n;
+        self.cursor = self.cursor.add(n);
+        debug_assert!(self.cursor <= self.end, "overflow");
     }
 
     #[inline]
     pub fn len(&self) -> usize {
-        self.slice.len()
+        self.end as usize - self.cursor as usize
     }
 
     #[inline]
     pub fn slice(&mut self) -> &'a [u8] {
         // not moving position at all, so it's safe
-        unsafe {
-            self.slice_skip(0)
-        }
+        let slice = unsafe { slice_from_ptr_range(self.start, self.cursor) };
+        self.commit();
+        slice
     }
 
+    // TODO: this is an anti-pattern, should be removed
     #[inline]
     pub unsafe fn slice_skip(&mut self, skip: usize) -> &'a [u8] {
-        debug_assert!(self.pos >= skip);
-        let head_pos = self.pos - skip;
-        let ptr = self.slice.as_ptr();
-        let head = slice::from_raw_parts(ptr, head_pos);
-        let tail = slice::from_raw_parts(ptr.add(self.pos), self.slice.len() - self.pos);
-        self.pos = 0;
-        self.slice = tail;
+        debug_assert!(self.cursor.sub(skip) >= self.start);
+        let head = slice_from_ptr_range(self.start, self.cursor.sub(skip));
+        self.commit();
         head
     }
+
+    #[inline]
+    pub fn commit(&mut self) {
+        self.start = self.cursor
+    }
 
     #[inline]
     pub unsafe fn advance_and_commit(&mut self, n: usize) {
-        debug_assert!(self.pos + n <= self.slice.len(), "overflow");
-        self.pos += n;
-        let ptr = self.slice.as_ptr();
-        let tail = slice::from_raw_parts(ptr.add(n), self.slice.len() - n);
-        self.pos = 0;
-        self.slice = tail;
+        self.advance(n);
+        self.commit();
+    }
+
+    #[inline]
+    pub fn as_ptr(&self) -> *const u8 {
+        self.cursor
+    }
+
+    #[inline]
+    pub fn start(&self) -> *const u8 {
+        self.start
+    }
+
+    #[inline]
+    pub fn end(&self) -> *const u8 {
+        self.end
+    }
+
+    #[inline]
+    pub unsafe fn set_cursor(&mut self, ptr: *const u8) {
+        debug_assert!(ptr >= self.start);
+        debug_assert!(ptr <= self.end);
+        self.cursor = ptr;
     }
 }
 
 impl<'a> AsRef<[u8]> for Bytes<'a> {
     #[inline]
     fn as_ref(&self) -> &[u8] {
-        &self.slice[self.pos..]
+        unsafe { slice_from_ptr_range(self.cursor, self.end) }
     }
 }
 
+#[inline]
+unsafe fn slice_from_ptr_range<'a>(start: *const u8, end: *const u8) -> &'a [u8] {
+    debug_assert!(start <= end);
+    core::slice::from_raw_parts(start, end as usize - start as usize)
+}
+
 impl<'a> Iterator for Bytes<'a> {
     type Item = u8;
 
     #[inline]
     fn next(&mut self) -> Option<u8> {
-        if self.slice.len() > self.pos {
-            let b = unsafe { *self.slice.get_unchecked(self.pos) };
-            self.pos += 1;
-            Some(b)
+        if self.cursor < self.end {
+            // SAFETY: bounds checked
+            unsafe {
+                let b = *self.cursor;
+                self.bump();
+                Some(b)
+            }
         } else {
             None
         }