rust: Refactor Rust impl of protover_all_supported().

This includes differences in behaviour to before, which should now more closely
match the C version:

 - If parsing a protover `char*` from C, and the string is not parseable, this
   function will return 1 early, which matches the C behaviour when protocols
   are unparseable.  Previously, we would parse it and its version numbers
   simultaneously, i.e. there was no fail early option, causing us to spend more
   time unnecessarily parsing versions.

 * REFACTOR `protover::ffi::protover_all_supported()` to use new types and
   methods.

src/rust/protover/ffi.rs

@@ -56,19 +56,26 @@ pub extern "C" fn protover_all_supported(
         Err(_) => return 1,
     };
 
-    let (is_supported, unsupported) = all_supported(relay_version);
+    let relay_proto_entry: UnvalidatedProtoEntry = match relay_version.parse() {
+        Ok(n)  => n,
+        Err(_) => return 1,
+    };
+    let maybe_unsupported: Option<UnvalidatedProtoEntry> = relay_proto_entry.all_supported();
 
-    if unsupported.len() > 0 {
-        let c_unsupported = match CString::new(unsupported) {
+    if maybe_unsupported.is_some() {
+        let unsupported: UnvalidatedProtoEntry = maybe_unsupported.unwrap();
+        let c_unsupported: CString = match CString::new(unsupported.to_string()) {
             Ok(n) => n,
             Err(_) => return 1,
         };
 
         let ptr = c_unsupported.into_raw();
         unsafe { *missing_out = ptr };
+
+        return 0;
     }
 
-    return if is_supported { 1 } else { 0 };
+    1
 }
 
 /// Provide an interface for C to translate arguments and return types for