Fix lifetime warning in Rust 1.70 (#352)

* Fix lifetime warning in Rust 1.70

Starting with rustc 1.70.0, the code in raw.rs:hash_get_multi produced the
following warnings:

    this `CString` is deallocated at the end of the statement, bind it to a
    variable to extend its lifetime

While it would work in practice due to the underlying implementation just
doing pointer juggling, this warning was correct: The CString created within
the macro `f` would be destroyed before the reference to the pointer taken
from it with `as_ptr` was used.

Resolved by binding the lifetime of the CStrings to that of the iterator
variable, which is alive until the function returns.

* Proper fix this time (hopefully)

src/raw.rs

@@ -473,6 +473,11 @@ where
 {
     assert_eq!(fields.len(), values.len());
 
+    let fields = fields
+        .iter()
+        .map(|e| CString::new(e.clone()))
+        .collect::<Result<Vec<CString>, _>>()?;
+
     let mut fi = fields.iter();
     let mut vi = values.iter_mut();
 
@@ -491,9 +496,7 @@ where
     }
     macro_rules! f {
         () => {
-            CString::new((*fi.next().unwrap()).clone())
-                .unwrap()
-                .as_ptr()
+            fi.next().unwrap().as_ptr()
         };
     }
     macro_rules! v {