Add string hashing support for dict keys

[antocuni]

This text/code has been written by claude following my precise instructions, and has been reviewed by me.

---

Summary

This PR adds hashing support for strings, enabling them to be used as dictionary keys. The implementation includes a cached hash value in the string structure and uses the FNV-1a hashing algorithm.

Key Changes

• String structure update: Added int32_t hash field to spy_Str struct to cache computed hash values
• Hash function implementation: Implemented spy_str_hash() using FNV-1a algorithm with special handling for edge cases (hash values of 0 and -1 are remapped to avoid collisions with uninitialized state)
• VM integration: Added w_hash_str() builtin function in the VM to expose string hashing to the language
• Memory layout fix: Updated string data offset from 4 to 8 bytes to account for the new hash field (size_t=8 bytes on 64-bit systems)
• Test coverage: Added test_str_keys() to verify dictionaries can use strings as keys

Implementation Details

• Hash values are computed lazily and cached in the string object on first access
• The hash field is initialized to 0 during string allocation, allowing detection of uncomputed hashes
• Special values -1 and 0 are remapped to -2 and 1 respectively to preserve the invariant that 0 means "not yet computed"
• The FNV-1a constants used are the standard 32-bit variants (offset basis: 2166136261, prime: 16777619)

https://claude.ai/code/session_014btrxaUSHiKYTzgR8dJxQy

[kanin-kearpimy]

Look good to me.