reduce FnArg::Regular variant size (#5039)

[luxedo]

Fixes #5039

I boxed the two fields @ngoldbaum suggested. New struct size:

print-type-size type: `method::RegularArg<'_>`: 56 bytes, alignment: 8 bytes
print-type-size     field `.from_py_with`: 8 bytes
print-type-size     field `.default_value`: 8 bytes
print-type-size     field `.option_wrapped_type`: 8 bytes
print-type-size     field `.ty`: 8 bytes
print-type-size     field `.name`: 24 bytes