Fixes #18

frb_check does exactly the same: `n > frb.len`

To support illumos based distributions the endian.h could be included on the specified location.

A regression from #18.

Otherwise we get compilation warnings:

```
asyncpg/pgproto/pgproto.c:23994:18: warning: ‘__pyx_f_7asyncpg_7pgproto_7pgproto_json_decode’ defined but not used [-Wunused-function]
23994 | static PyObject *__pyx_f_7asyncpg_7pgproto_7pgproto_json_decode(struct __pyx_obj_7asyncpg_7pgproto_7pgproto_CodecContext *__pyx_v_settings, struct __pyx_t_7asyncpg_7pgproto_7pgproto_FRBuffer *__pyx_v_buf) {
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
asyncpg/pgproto/pgproto.c:23886:18: warning: ‘__pyx_f_7asyncpg_7pgproto_7pgproto_json_encode’ defined but not used [-Wunused-function]
23886 | static PyObject *__pyx_f_7asyncpg_7pgproto_7pgproto_json_encode(struct __pyx_obj_7asyncpg_7pgproto_7pgproto_CodecContext *__pyx_v_settings, struct __pyx_obj_7asyncpg_7pgproto_7pgproto_WriteBuffer *__pyx_v_buf, PyObject *__pyx_v_obj) {
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Use `str` instead. 
Source: 
- https://github.com/python/cpython/blob/35b5eaa176a5bae8a0cacb9c9f40ec948ecc4325/Lib/typing.py#L3419-L3420
- python/cpython#92332

ReadBuffer.redirect_messages now returns the number of messages redirected.
This is needed for counting RowDescription messages to figure out
how many rows were modified by DML statements.

I'll be sending a fix upstream also but I want to move things forward
on our side without waiting for a Cython release.

Move some definitions into `buffer.{pyx,pxd}` to reduce the hassle of
cimporting it.

This makes it so that Cython can store it in the module state