fix: support pointer key types in hashKey

[AdamMagued]

Summary

In Go, native maps and sync.Map support pointer keys. In cornelk/hashmap, instantiating or hashing maps with pointer keys (*T or unsafe.Pointer) was unsupported because the hashable type constraint only allowed numeric and string types, and setDefaultHasher did not handle reflect.Pointer or reflect.UnsafePointer, causing a panic.

Changes

• Updated the hashable interface constraint in defines.go to comparable, allowing pointer types while preserving compile-time comparability requirements.
• Added reflect.Pointer and reflect.UnsafePointer to setDefaultHasher in util_hash.go, hashing the pointer address value using word-size xxhash (xxHashQword on 64-bit systems, xxHashDword on 32-bit systems).
• Added regression tests in hashmap_test.go verifying insertion, retrieval, overwrite, deletion, and nil pointer handling for struct pointer keys (*struct{ name string }) and unsafe.Pointer keys.
• Added unit tests in util_hash_test.go verifying pointer and unsafe.Pointer hashing behavior against the underlying word hasher.

Fixes #53