`METH_METHOD` calling convention is now not so efficient

Bug report

Bug description:

There are callables implemented with the METH_METHOD|METH_FASTCALL signature in C. They can be 5%-15% less efficient than using only METH_FASTCALL (or METH_O) with a PyType_GetModuleByDef function call.

For example, I measured the difference on Windows PGO builds by duplicating functions:

• CDataType_from_buffer_copy() in _ctypes.c, which is not called when profiling:

  from timeit import timeit
  setup = """if 1:
      import ctypes
      buf = bytearray(16)
      cls = ctypes.c_char * len(buf)
  """
  # with a warmup
  for _ in range(2):
      # METH_METHOD|METH_FASTCALL (as-is)
      r0 = timeit(s0 := f'cls.from_buffer_copy (buf)', setup)
  
      # METH_FASTCALL (no `defining_class`) + PyType_GetModuleByDef
      r1 = timeit(s1 := f'cls.from_buffer_copy1(buf)', setup)
  
  print(s0, r0, 1 + (1 - r0 / r0))
  print(s1, r1, 1 + (1 - r1 / r0))
  

  cls.from_buffer_copy (buf) 0.15552800190635024 1.0
  cls.from_buffer_copy1(buf) 0.13187471489945893 1.1520837837364741
  

• dec_mpd_qquantize() in _decimal.c profiled with 6800 calls (unfair?):

  # legacy (as-is)
  d1.quantize (d2) 0.1694609627971658 1.0
  
  # METH_METHOD|METH_FASTCALL (`defining_class`) + _PyType_GetModuleState
  d1.quantize1(d2) 0.1408861404022900 1.168621857938327
  
  # METH_FASTCALL (no `defining_class`) + PyType_GetModuleByDef
  d1.quantize2(d2) 0.1258157708973158 1.257553074049807
  

  Script (expand)

  from timeit import timeit
  setup = """if 1:
      from _decimal import Decimal
      d1,d2 = Decimal(1.414), Decimal('0.01')
  """
  for _ in range(2):
      r0 = timeit(s0 := f'd1.quantize (d2)', setup)
      r1 = timeit(s1 := f'd1.quantize1(d2)', setup)
      r2 = timeit(s2 := f'd1.quantize2(d2)', setup)
  
  print(s0, r0, 1 + (1 - r0 / r0))
  print(s1, r1, 1 + (1 - r1 / r0))
  print(s2, r2, 1 + (1 - r2 / r0))
  

Observations:

• The number of arguments had little to do with this.
• The gaps seem to be consistent as long as they are equally (un)exercised.
• The same goes for non-PGO builds and builtin modules (e.g. _sre), where the impacts may be less significant.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Start by reproducing the reported benchmarks for CDataType_from_buffer_copy() in _ctypes.c and dec_mpd_qquantize() in _decimal.c, comparing METH_METHOD|METH_FASTCALL with METH_FASTCALL and the module lookup calls shown. Read the METH_METHOD and PyType_GetModuleByDef entry points, then define and validate an accepted approach whose performance is no worse than the alternatives across the cited cases.