BUG: Handle iso_c_type mappings more consistently

[charris]

Backport of #25226.

Closes #25207. This one could use a backport. Needs:

• Scoping TODOs (can't rewrite the entire cfuncs here)
• Tests
• Check that SciPy builds

As discussed below, there were basically two deficiencies:

• Type maps were not propagated
• Fortran types (iso_c_binding) were mapped to ignored C types
  • This is the default behavior, but is silent and the maps are done to basic (lowest precision) corresponding types

This PR fixes (pragmatically) both these deficiences, for the first problem, the fix is a simple refactor and cleanup.

The second issue is a little trickier. For now, the pragmatic solution (as done by users anyway) is to map the iso_c_binding kinds to supported f2py types, i.e. to one of:

c2py_map = {'double': 'float',
            'float': 'float',                          # forced casting
            'long_double': 'float',                    # forced casting
            'char': 'int',                             # forced casting
            'signed_char': 'int',                      # forced casting
            'unsigned_char': 'int',                    # forced casting
            'short': 'int',                            # forced casting
            'unsigned_short': 'int',                   # forced casting
            'int': 'int',                              # forced casting
            'long': 'int',
            'long_long': 'long',
            'unsigned': 'int',                         # forced casting
            'complex_float': 'complex',                # forced casting
            'complex_double': 'complex',
            'complex_long_double': 'complex',          # forced casting
            'string': 'string',
            'character': 'bytes',
            }

Pending a more complete overhaul of the generated bindings (tracked in #25229, which @Pranavchiku is also looking into) this is the optimal bugfix for #25207.