Summary

This PR collects together all of the macro constants associated with the "typecode" value used across several different modules to encode information about the type identity of an associated pointer or memory buffer. These values were previously scattered across the codebase, often leading to confusion about the C API, resulting in e.g. defective memoryview objects with no typecode except for the read-write flag.

With this change:

• Anyone searching for information about the typecode values is led to a single location where they are all listed.
• Other aliases for the same types can be used transparently: For instance, MP_TYPECODE_C(custom_typedefed_int_t) would automatically pick the appropriate typecode letter at compile time thanks to the semantics of _Generic.
• Instances where an incorrect association from typecode to C type have been made will be a lot more obvious to inspection. For instance, take the following bug:

  case 'q':
      long double *x = obj;
      return quad_float_op(*x);

  Maybe you already know reflexively that that's NOT actually what 'q' means, but it's still a lot easier to spot if you instead write it like:

  case MP_TYPECODE_C(long long int):
      long double *x = obj;
      return quad_float_op(*x);

Testing

I've run the test suite across unix and rp2 on my own hardware. All extant tests pass; though as a pure refactor, this change can't be checked directly.

Some of the changes touch port-specific code for devices I don't have access to; hopefully CI will catch anything aggressively defective, but this ought to be tested on those other devices.

Trade-offs and Alternatives

Using _Generic limits compatibility to C11 and above. If that's a problem, potentially separate macros for each type could be declared instead --- but that's only marginally useful than single-letter typecodes.

Potentially, it would be better to use the stdint names for the typecodes, i.e. uint8_t instead of unsigned char; and in several of the places I've refactored there might be a more semantic typename to use as the argument --- the actual typename of the associated buffer, or perhaps even a typeof expression involving that buffer.