feat(stable-api): Add RB_ID2SYM, RB_SYM2ID for Symbol/ID conversions

[ianks]

Summary

Add stable API methods for converting between Symbols and IDs, equivalent to RB_ID2SYM and RB_SYM2ID.

Motivation

Symbol/ID conversions are fundamental operations in Ruby extensions for:

• Method dispatch and respond_to? checks
• Hash key handling (symbols are common hash keys)
• Attribute accessors

While rb_id2sym() and rb_sym2id() functions exist, the macro versions are more efficient for static symbols.

Proposed API

/// Convert an ID to a Symbol VALUE (akin to `RB_ID2SYM`).
///
/// Returns a static Symbol VALUE for the given ID.
fn id2sym(&self, id: ID) -> VALUE;

/// Convert a Symbol VALUE to an ID (akin to `RB_SYM2ID`).
///
/// # Safety
/// The caller must ensure the VALUE is actually a Symbol.
/// For dynamic symbols, this may need to access heap data.
unsafe fn sym2id(&self, obj: VALUE) -> ID;

Implementation Notes

• Static symbols use tagged pointers: (id << 8) | 0x0c
• Dynamic symbols (Ruby 2.2+) are heap-allocated
• Need to handle both static and dynamic symbol cases
• Reference: include/ruby/internal/symbol.h

Checklist

• Add methods to StableApiDefinition trait
• Implement for each Ruby version
• Handle static vs dynamic symbols
• Add C fallback in compiled.c
• Add public macro wrappers in macros.rs
• Add tests for both static and dynamic symbols