make Parser and Packer functions consistent and reciprocal

[coolaj86]

• Some of the functions return a whole message, pre-wrapped
• Others require wrapping after the fact
• (they should all be the same)
• Most aren't reciprocal either
• (but they should be)

let obj = Parser.parseXyz(bytes);
let sameBytes = Packer.packXIyz(obj);
assert.equal(bytes, sameBytes);