Migrating `#[pyproto]` to `#[pymethods]`

[davidhewitt]

This is a tracking issue for the work remaining to complete the support for __dunder__ (aka "protocol") methods in #[pymethods].

As per the proposal in #1864, the current intention is to have a working but experimental implementation of this in 0.15. If any feedback from this implementation is ok, I'd suggest to deprecate #[pyproto] in 0.16 or 0.17. If we're happy with the whole implementation earlier / later we can always adjust these aims.

The items outstanding after #1864

• Documentation
• Some protocols which need design discussion:
  • The GC protocol - doesn't have a well-specified __dunder__ at the Python level, so we'll need to design one
  • The buffer protocol - same thing for this
  • The sequence protocol - it overlaps messily with the mapping protocol; we might want to do the same as what happens at the Python level, or we might want to make __sequence_getitem__ methods etc to be explicit. See also Allow taking slices when implementing PySequenceProtocol __getitem__ #1855
• Suppport for multiple-pymethods feature
• tests for
  • __delete__
  • __anext__
  • __aiter__
  • __index__
  • __int__
  • __float__
  • __invert__
• (maybe) better argument extraction errors (if there's an error, the generated code won't say which argument failed, athough that's probably obvious as most these methods take at most one argument).
• __call__
• __new__ / __init__ ?
• __getitem__ iteration. #1927 ?