IOPattern from and to bytes for portability

[alxiong]

Fix #28

cc @mmaker

[alxiong]

I have updated the PR to address my follow-up comment: #28 (comment) @mmaker

[mmaker]

Hey, thank you so much. Some points:

• I'm not convinced we need to implement equality checks for IOPattern (this seems just for the test)
• Given that we have .as_bytes I don't think we need .to_bytes as well. Overall, given that we're just doing hashing, it would be great to rely only on stack allocations in the core library
• I don't know if it from_bytes should rather be a AsRef<[u8]> or a Read implementation, given the length of these functions.

For now I'm leaning on thinking it's just simpler to expose from_string(): if you have already to share metadata between prover and verifier, you already have to think about encodings and strings.
I'm interested in hearing if this complicates things on your end and I misunderstood something.

For reference, your ideas on how to implement Safe::preprocess() would be super valuable.
I exposed from_string in 857289b

[alxiong]

all sounds reasonable. closing this now.
thanks a lot!