feat(stable-api): Add FIX2LONG, LONG2FIX for Fixnum conversions

[ianks]

Summary

Add stable API methods for converting between Fixnum VALUEs and C long integers, equivalent to the C macros FIX2LONG, FIX2ULONG, LONG2FIX, etc.

Motivation

Fixnum conversion is one of the most common operations in Ruby C extensions. The macros are simple bit shifts but having them in the stable API ensures:

• Consistent behavior across Ruby versions
• No need for users to understand the internal representation
• Type safety through Rust's type system

Proposed API

/// Convert a Fixnum VALUE to a long (akin to `FIX2LONG`).
///
/// # Safety
/// The caller must ensure the VALUE is actually a Fixnum (use FIXNUM_P first).
#[inline(always)]
fn fix2long(&self, obj: VALUE) -> c_long;

/// Convert a Fixnum VALUE to an unsigned long (akin to `FIX2ULONG`).
///
/// # Safety  
/// The caller must ensure the VALUE is actually a Fixnum.
#[inline(always)]
fn fix2ulong(&self, obj: VALUE) -> c_ulong;

/// Convert a long to a Fixnum VALUE (akin to `LONG2FIX`).
///
/// # Note
/// This does NOT check if the value fits in a Fixnum range.
/// Use `rb_int2inum` for arbitrary integers.
#[inline(always)]
fn long2fix(&self, val: c_long) -> VALUE;

/// Convert an unsigned long to a Fixnum VALUE (akin to `ULONG2NUM`).
#[inline(always)]
fn ulong2num(&self, val: c_ulong) -> VALUE;

/// Check if a long fits in a Fixnum (akin to `FIXABLE`).
#[inline(always)]
fn fixable(&self, val: c_long) -> bool;

/// Check if an unsigned long fits in a Fixnum (akin to `POSFIXABLE`).
#[inline(always)]
fn posfixable(&self, val: c_ulong) -> bool;

Implementation Notes

• Fixnum uses tagged pointers: (val << 1) | 1
• Range is approximately -(2^62) to (2^62 - 1) on 64-bit
• These are simple bit operations, very fast
• Reference: include/ruby/internal/arithmetic/fixnum.h

Checklist

• Add methods to StableApiDefinition trait
• Implement for each Ruby version
• Add C fallback in compiled.c
• Add public macro wrappers in macros.rs
• Add comprehensive tests including edge cases
• Add to show-asm script for performance verification