Move aws_byte_buf_contents_match to AWS C-common

[adpaco-aws]

As discussed in #638 , it makes sense to have this function in AWS C-common instead. It may be the case that we also want to move key_contents_match (pending discussion).

[feliperodri]

Let's not move key_contents_match to AWS C Common, because it uses data structures from ESDK-C. We should move all utility functions that manipulate only AWS C Common data structures.