Specialize DefPathHash table to skip crate IDs

Instead, we store just the local crate hash as a bare u64. On decoding,
we recombine it with the crate's stable crate ID stored separately in
metadata. The end result is that we save ~8 bytes/DefIndex in metadata
size.

One key detail here is that we no longer distinguish in encoded metadata
between present and non-present DefPathHashes. It used to be highly
likely we could distinguish as we used DefPathHash::default(), an
all-zero representation. However in theory even that is fallible as
nothing strictly prevents the StableCrateId from being zero.

compiler/rustc_span/src/def_id.rs

@@ -114,8 +114,6 @@ impl DefPathHash {
     }
 
     /// Returns the crate-local part of the [DefPathHash].
-    ///
-    /// Used for tests.
     #[inline]
     pub fn local_hash(&self) -> Hash64 {
         self.0.split().1