Use separate types per path entry.

Previously `mc-sgx-ql::PathInitializer::with_paths()` used the same
generic type for all the pathlike parameters. This had the side effect that all four
paths had to be the same type. Now each pathlike parameter has it's own
type.

dcap/ql/src/quote_enclave.rs

@@ -48,7 +48,7 @@ impl PathInitializer {
         Self::with_paths(
             "/usr/lib/x86_64-linux-gnu/libsgx_qe3.signed.so.1",
             "/usr/lib/x86_64-linux-gnu/libsgx_pce.signed.so.1",
-            None,
+            None::<&Path>,
             "/usr/lib/x86_64-linux-gnu/libsgx_id_enclave.signed.so.1",
         )
     }
@@ -73,12 +73,18 @@ impl PathInitializer {
     ///     * one of the paths does not point to a file
     ///     * one of the paths is longer than 259 (bytes)
     ///     * one of the paths contains a null (0) byte.
-    pub fn with_paths<P: AsRef<Path>>(
-        quoting_enclave: P,
-        provisioning_certificate_enclave: P,
-        quote_provider_library: Option<P>,
-        id_enclave: P,
-    ) -> Result<()> {
+    pub fn with_paths<P1, P2, P3, P4>(
+        quoting_enclave: P1,
+        provisioning_certificate_enclave: P2,
+        quote_provider_library: Option<P3>,
+        id_enclave: P4,
+    ) -> Result<()>
+    where
+        P1: AsRef<Path>,
+        P2: AsRef<Path>,
+        P3: AsRef<Path>,
+        P4: AsRef<Path>,
+    {
         let mut value = PATH_INITIALIZER.lock().expect("Mutex has been poisoned");
         if value.is_none() {
             Self::set_paths(
@@ -110,12 +116,18 @@ impl PathInitializer {
         }
     }
 
-    fn set_paths<P: AsRef<Path>>(
-        quoting_enclave: P,
-        provisioning_certificate_enclave: P,
-        quote_provider_library: Option<P>,
-        id_enclave: P,
-    ) -> Result<PathInitializer> {
+    fn set_paths<P1, P2, P3, P4>(
+        quoting_enclave: P1,
+        provisioning_certificate_enclave: P2,
+        quote_provider_library: Option<P3>,
+        id_enclave: P4,
+    ) -> Result<PathInitializer>
+    where
+        P1: AsRef<Path>,
+        P2: AsRef<Path>,
+        P3: AsRef<Path>,
+        P4: AsRef<Path>,
+    {
         Self::set_path(PathKind::QuotingEnclave, quoting_enclave)?;
         Self::set_path(
             PathKind::ProvisioningCertificateEnclave,