gh-113317: Argument Clinic: Add libclinic.converters module (#117315)

Move the following converter classes to libclinic.converters:

* PyByteArrayObject_converter
* PyBytesObject_converter
* Py_UNICODE_converter
* Py_buffer_converter
* Py_complex_converter
* Py_ssize_t_converter
* bool_converter
* byte_converter
* char_converter
* defining_class_converter
* double_converter
* fildes_converter
* float_converter
* int_converter
* long_converter
* long_long_converter
* object_converter
* self_converter
* short_converter
* size_t_converter
* slice_index_converter
* str_converter
* unicode_converter
* unsigned_char_converter
* unsigned_int_converter
* unsigned_long_converter
* unsigned_long_long_converter
* unsigned_short_converter

Move also the following classes to libclinic.converters:

* buffer
* robuffer
* rwbuffer

Move the following functions to libclinic.converters:

* correct_name_for_self()
* r()
* str_converter_key()

Move Null and NULL to libclinic.utils.