hashlib: Refactor, split, and optimise.

[jimmo]

This splits out each algorithm into its own extension package, so that only the necessary algorithms can be installed.

This allows for a significant reduction in RAM and flash. i.e. previously installing hashlib meant that all algorithms were imported.

Additionally ensures that any built-in hash algorithms (from uhashlib) are still exposed (e.g. md5), and retains the existing behavior to use the built-in preferentially. (Replaces #674)

Also includes a refactoring of the algorithms to reduce code size and reduce the number of allocations they do as well as using bytearrays in place of list-of-int where possible.

Add more comprehensive tests (using unittest).

This work was funded through GitHub Sponsors.

[jimmo]

For example, if you previously only wanted, say sha512, the package would install

 317 __init__.mpy
  90 _sha224.mpy
6157 _sha256.mpy
  90 _sha384.mpy
8692 _sha512.mpy

and all of this code would be resident (15346 bytes of .mpy files total). Even if you already had built-in sha256, it still ends up importing sha256 to provide sha224.

Now you only install and load the files you need (which are also each smaller):

  321 __init__.mpy
  272 _sha224.mpy
 4889 _sha256.mpy
  367 _sha384.mpy
 6983 _sha512.mpy
  557 _sha.mpy

(Note that __init__.py and _sha.py are always required, and _sha224.py uses _sha256.py, and _sha384.py uses _sha512.py)

So now if you just want to add sha512, you can just install hashlib-sha512 and it will install and load just 7861 bytes of .mpy files.

[KipCrossing]

@jimmo What else needs doing before this can be merged?

[dpgeorge]

I removed the now-empty test_hashlib.py file and rebased on master.

[dpgeorge]

Thanks for this, it's a very nice improvement.