gh-135004: rewrite and cleanup `blake2module.c` (#135006)

* Cleanup imports and update module docstring.
* Simplify detection of SIMD support.
* Correctly guard `update()` cases.
* Rewrite `py_blake2b_or_s_new` and rename it to `py_blake2_new`.
* Rewrite `blake2_blake2b_copy_locked` and `py_blake2_clear`.
* Refactor computations of `digest` and `hexdigest`.
* Simplify `py_blake2b_get_name` and `py_blake2b_get_block_size`.
* Add `hacl_get_blake2_info` to extract static BLAKE-2 information.
   This new helper is used by `py_blake2b_get_digest_size`, but can
   be later used to expose `key_length` more easily.