marshal: recover loads regression with raw refs table + tagged state

Replace the PyList-backed reference table with a raw growable
PyObject ** array, and encode REF_STATE_INCOMPLETE_HASHABLE in the low
bit of each ref pointer so the parallel state-byte allocation is gone.

Also:
- drop the allow_incomplete_hashable parameter from r_object; it lives
  on RFILE now, auto-reset on entry, flipped via a wrapper at the two
  list-element / dict-value sites.
- force-inline the r_ref_* helpers so the compiler can fold the
  if (flag) guards into the callers as the original R_REF macro did.

Misc/marshal-perf-diary.md records the full experiment ledger: each
idea tested in isolation, results, and the combined stack. Benchmark
harness is /tmp/marshal_bench_cpu_stable.py (200k loads x 11 repeats,
taskset -c 0, best-of-3 pinned-run median).

Combined deltas vs main on loads:

    small_tuple    14.3% faster
    nested_dict     6.9% faster
    code_obj        6.8% faster

dumps is roughly flat to slightly faster. test_marshal passes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mjbommar