feat!: Optimize JSON key deserialization with arena-based interning

- Add KeyArena in src/string.rs for per-document string deduplication
- Refactor Value deserialization to intern object keys via ValueSeed
- Ensure no_std compatibility with hashbrown + FxHasher
- Add unit test confirming Rc<str> reuse across deserialized objects
- Update Cargo.toml with hashbrown and rustc-hash dependencies

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

Author: anakrish

Cargo.lock

@@ -117,7 +117,9 @@ rand = { version = "0.9.0", default-features = false, features = ["thread_rng"],
 # Causes the project to link with the Spectre-mitigated CRT and libs.
 msvc_spectre_libs = { version = "0.1", features = ["error"], optional = true }
 dashmap = { version = "6.1", default-features = false, optional = true }
+hashbrown = { version = "0.15", default-features = false }
 mimalloc = { path = "mimalloc", optional = true }
+rustc-hash = { version = "1.1", default-features = false }
 
 [dev-dependencies]
 anyhow = "1.0.45"