Optional acknowledgements of success / failure for packets

[cwgoes]

Split off from #126.

• Acknowledgements can be enabled as an option on a channel.
• Acknowledgements that would be no-ops need not be actually executed.

[cwgoes]

Simple model:

• On packet execution, application can return opaque bytestring which the IBC handler will store a commitment to

Questions:

• Should application-level failures close ordered channels or should applications close manually?
• Does the IBC protocol itself need to understand the acknowledgement data (status bit for success / failure) or can it be opaque?

[cwgoes]

Instead of having acknowledgements be an option on the channel I think the receiving module can just choose to write empty data.

[cwgoes]

For now - modules will just close channels as normal. We need to think more about the closing handshake and what happens when modules misbehave in the middle of it.