Add a dedicated length-prefixing method to Hasher

This accomplishes two main goals:
- Make it clear who is responsible for prefix-freedom, including how they should do it
- Make it feasible for a `Hasher` that *doesn't* care about Hash-DoS resistance to get better performance by not hashing lengths

This does not change rustc-hash, since that's in an external crate, but that could potentially use it in future.

compiler/rustc_data_structures/src/stable_hasher.rs

@@ -73,6 +73,17 @@ impl Hasher for StableHasher {
         self.state.write(bytes);
     }
 
+    #[inline]
+    fn write_str(&mut self, s: &str) {
+        self.state.write_str(s);
+    }
+
+    #[inline]
+    fn write_length_prefix(&mut self, len: usize) {
+        // Our impl for `usize` will extend it if needed.
+        self.write_usize(len);
+    }
+
     #[inline]
     fn write_u8(&mut self, i: u8) {
         self.state.write_u8(i);