Use standard type ssize_t to define HPy_ssize_t.

[fangerer]

This is the follow-up issue of #343 .

We aim to use fixed-width integers in the ABI to improve compatibility and portability. However, HPy assumes in several and crucial places that sizeof(HPy_ssize_t) == sizeof(Py_ssize_t).

In February 2023's dev call, we decided to not use a fixed-width integer like int64_t for HPy_ssize_t now since it is a bit more involved.

Instead, we will use do typedef ssize_t HPy_ssize_t for now.