Get pedersen + transcripts fully working, with tests

[laudiacay]

This will give you a good look into the transcripts, fiat-shamir, and pedersen commitments. I don't think it'll be too hard, most of this except the pedersen is done.

• Implement all the todos in the pedersen commitments module: commit_native_with_multiple_indices, commit_native, compress_native_array, compress_native_buffer_to_field, compress_native, compress_native_with_multiple_indices, compress_native_index, merkle_damgard_tree_compress, merkle_damgard_compress_with_multiple_ivs, merkle_damgard_compress
• Implement all the tests: zero_one, endomorphism_test, hash_single, hash_pair, merkle_damgard_compress, merkle_damgard_compress_multiple_iv, merkle_damgard_tree_compress. Get them passing.
• Make sure all three hasher types are good to go and fully implemented, producing sane output vs original bberg (is the way the hash functions are being split sane, in your opinion? feel free to workshop it... I think it's okay but not fantastic...)
• get the univariate_serialization and validate_transcript tests written and passing

After this, the batch_commit test from the commitments bounty should be tackle-about.