Add support for decoding instructions to the client gen

[nhruo123]

I have a use case in which I need to decode a stream of bytes and an array of accounts into the generated args and accounts of an instruction.

I would like to have the following additions to the client gen:

1. Export the identifier of each instruction.
2. Add a decode_<instruction_name> function that accepts bytes and Sequence[Pubkey] and returns a tuple of <instruction_name>Args and <instruction_name>Accounts (or better yet a class that wraps both the args and accounts to a single type).
3. A general decode function that accepts bytes and Sequence[Pubkey] and calls a decode_X based on the identifier at the start of the byte array that returns the value of the decode function.