Move the pure-Python implementation of difflib to Lib/_pydifflib.py and
turn Lib/difflib.py into a thin shim that re-exports its public API.
This mirrors the layout used by decimal/_pydecimal, datetime/_pydatetime,
and pickle/_pickle, where the public module dispatches to a faster C
implementation when available and the pure-Python module is preserved
as a self-contained reference for alternative Python implementations.

No public behaviour change.  ``Match`` is constructed with
``module='difflib'`` so its qualified name matches the public module.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Introduce Modules/_difflibmodule.c, a heap-type C extension that
implements __init__, set_seqs/set_seq1/set_seq2, find_longest_match,
get_matching_blocks, get_opcodes, and ratio for SequenceMatcher.  The
inner DP loop and the full Ratcliff-Obershelp recursion run on int32
label arrays with zero Python C-API calls in the hot path; codepoint-
keyed lookup tables short-circuit per-element dict probes for str and
bytes inputs.  Output is bit-identical to the pure-Python implementation
including tie-breaks.

Lib/difflib.py grows a small subclass that inherits the slow-path
methods (quick_ratio, real_quick_ratio, get_grouped_opcodes) from the
pure-Python class; this is a no-op when the accelerator is not built.

Build wiring: configure.ac registers the module via PY_STDLIB_MOD_SIMPLE
and Modules/Setup.stdlib.in references _difflibmodule.c.  configure must
be regenerated with autoreconf before this lands.

Typical workloads run 5-25x faster than pure Python; the bytes path up
to ~70x.  See Lib/test/test_difflib.py for cross-implementation tests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>