[Toybox] patch: add built-in versions of sha-2 family hash functions

[Dan Brown]

Hello- here is a patch that provides these hash functions using built-in
routines instead of relying on the OpenSSL library.

sha224sum
sha256sum
sha384sum
sha512sum

Regarding the license, this is based on the wikipedia page for SHA-2
<https://en.wikipedia.org/wiki/SHA-2>, and doesn't use any existing code.
The example intermediate calculations from the NIST
<https://csrc.nist.gov/projects/cryptographic-standards-and-guidelines/example-values>
were helpful in debugging.

Comments and suggestions are welcome. Here are a few thoughts and questions
for the list:

Tests. Do I need to add some? Please point me to an example.

How to deal with endianness? How should I test the behavior of a big-endian
system when developing on a little-endian system?

I changed the data types to be explicitly sized (eg. uint32_t instead of
int). Or is this already taken care of as part of the portability functions?

SHA512 (and SHA384) are a bit tricky. I couldn't figure out how to
calculate the constants to the full 64-bit precision needed. Let me know if
there are data types >64bit and corresponding math functions (cube root and
floor) that are possible.

Dan
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