Implement value encoding and decoding

[virgil-serbanuta]

Depends on #135

We would need to add the following the Helpers trait: fn encodeU8(Bytes, u8) -> Bytes, fn encodeU16(Bytes, u16) -> Bytes ... to the Bytes type (these would append to the bytes object).

We would need to also add fn decodeU8(Bytes) -> (Bytes, u8, bool), ... - these would return the decoded value and the remaining Bytes, and a success flag.

[ACassimiro]

Addressed in #167 and #171.