Add methods for bloom filter compression

[quantumagi]

The purpose of this PR is to reduce the serialized size of the bloom filter used by the PosBlockHeader. Since the bloom filter is basically a bit map of 2048 bits contained in 256 bytes it is quite possible that many of those bytes may be 0. This compression scheme reduces the number of 0 bytes by storing the number of 0 bytes instead of the bytes themselves. As an example 256 0 bytes would be encoded as 1 255 0 - where 1 is the total compressed length - 1, followed by 255 zeros and finally one more zero. The general pattern is <compressed length - 1> [<number of zeros> <explicit byte>] x n where n > 0. The last explicit byte is added only if required. If the compressed length exceeds 255 then the bloom filter is not compressed when serialized, which is flagged by 255 in the first byte. A 0 in the first byte is not possible and is reserved for future use.

This is a backwards compatible change required by system contract and does not affect legacy bloom filters.

This PR is being split off from PR #450 to reduce the size of that PR.

[fassadlr]

@quantumagi can you detail what the requirement here is for the compression?

[quantumagi]

@fassadlr , Done.

[fassadlr]

@quantumagi merge this to #450 👍