Add sha256 function

[Acaccia]

This PR adds sha256 function that works on buffers and (u)int.

This is entirely written in Webassembly. I even used some SIMD whenever I could. I'm proud.

This is part of #69. sha256 is needed in order to write hash160.

[obycode]

This looks great! Thanks @Acaccia. Just one comment and also a request to add property tests as well.

[Acaccia]

@obycode any possibility to ask someone else to write the property tests here? I'm very focused on the implementation of hash160 right now.

There are a few complications to consider in this case:

• We should test it against the sha256 that is used in stacks-blockchain/clarity.
• We have to create a new Strategy for proptest to generate buffs.
• When I designed the property tests, I used a "huge hack" where I put the Store in a Refcell to hide its mutability to the compiler. This allowed us to create the pair (instance, store) and to load the tested function only once before launching the property test. In this case, we actually have to write to the memory inside the Store, so I don't know how the proptest crate will react to this.

Also, I would say that in this case, the testing of multiple input is not that important. A tiny drift in sha256 causes massive implications, and I believe that the current tests are enough in this case.

[obycode]

Following up from the meeting, I'm okay with getting this merged and saving the additional testing for later, with @csgui taking a look after his current work.

[obycode]

lgtmn!