feat: generate IBCHandler contract interface crate (#76)

Generate contract interface for IBCHandler, using ethers' abigen
functionality (note that currently my fork is being used as there was a
small issue with the generation). Also adds a stub in the relayer to use
said crate.

This branch is off of #26, once that's merged I'll rebase.

NOTE: This PR now also contains a basic implementation of a connection
and channel handshake (along with client creation on both sides).