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.

Author: picnixz

Misc/NEWS.d/next/Library/2025-06-01-14-18-48.gh-issue-135004.cq3-fp.rst

@@ -0,0 +1,3 @@
+Rewrite and cleanup the internal :mod:`!_blake2` module. Some exception
+messages were changed but their types were left untouched. Patch by Bénédikt
+Tran.