IF: Change bls_public_key & bls_signature serialization

[heifner]

Change bls_public_key and bls_signature serialization to be:

• Non-Montgomery form and little-endian encoding for the field elements.
• Affine form for the group element (the z component is 1 and not included in the serialization).
• Binary serialization encodes x component first followed by y component.

Add bls_aggregate_signature which is mutable version of bls_signature. bls_signature is now immutable.

Serialize as variable sized array and instead of a fixed sized array to make it easier to use in external tools that use ABI since our ABI does not handle fixed sized arrays.

Resolves #2334

[ericpassmore]

Note:start group: IF category: INTERNALS summary: Update internal storage format of BLS keys to optimize usage. No change to external representation of keys. Note:end