Add thread-local string interner for ReString

Introduces InternedReString, a wrapper around ReString that deduplicates
long strings via a per-thread HashSet. Strings longer than size_of::<String>()
are stored as Arc<str> and looked up on construction and during deserialization
(serde and bilrost), so repeated values share a single heap allocation.

Short strings are stored inline and bypass the interner entirely.

The bilrost decode path is optimized with a fast path for contiguous buffers:
when chunk() covers the full string, we read directly as &str and query the
interner without allocating a temporary buffer.

Author: AhmedSoliman

Cargo.lock

@@ -19,7 +19,9 @@ restate-workspace-hack = { workspace = true }
 bilrost = { workspace = true, optional = true }
 bytes = { workspace = true, optional = true }
 compact_str = { workspace = true, features = ["std"] }
+ahash = { workspace = true }
 serde = { workspace = true, optional = true }
+hashbrown = { workspace = true }
 
 [dev-dependencies]
 restate-util-string = {path = ".", default-features = false, features = ["bytes", "bilrost", "serde"]}