Remove message type bound on `ResponseInstruction`

Remove message type bound on `ResponseInstruction`

Our onion message handlers generally have one or more methods which
return a `ResponseInstruction` parameterized with the expected
message type (enum) of the message handler.

Sadly, that restriction is impossible to represent in our bindings -
our bindings concretize all LDK structs, enums, and traits into a
single concrete instance with generics set to our concrete trait
instances (which hold a jump table). This prevents us from having
multiple instances of `ResponseInstruction` structs for different
message types.

Our bindings do, however, support different parameterizations of
standard enums, including `Option`s and tuples.

In order to support bindings for the onion message handlers, we
are thus forced into std types bound by expected message types,
which we do here by making `ResponseInstruction` contain only the
instructions and generally using it as a two-tuple of
`(message, ResponseInstruction)`.