Adds a C-level fast path for serializing and deserializing Ruby Struct
types, bypassing Ruby proc callbacks entirely.

Usage:
  factory.register_type(0x01, MyStruct, optimized_struct: true)

Benefits:
- Directly accesses struct fields via RSTRUCT_GET in C
- No Ruby method calls for field access
- No proc allocation or invocation overhead
- Supports both regular and keyword_init structs

The wire format is identical to the recursive proc-based approach,
ensuring compatibility with existing serialized data.

This provides significant performance improvements for applications
that serialize many Struct objects.

When registering an extension type with ref_tracking: true, repeated
objects are serialized as back-references instead of being re-encoded.
This reduces payload size when the same object appears multiple times.

Uses ext type 127 with a compact wire format:
- New reference: nil marker followed by the serialized object
- Back-reference: positive integer ref_id pointing to earlier object

Very sligthly slower than naive positional, but work with
both positional and keyword structs.

Saves a few allocations. Mostly helps on smaller
payloads, as the cost of building the unpacker is quite high.