Redesign output for the sandbox

- Removed Output() from the Go sandbox interface as its only viable
action was to create a new message (duplicating inject_message())
- output() in the Lua sandbox API now writes data to a payload
buffer and when the payload is ready to be sent the sandbox calls
inject_message() which creates a new message and resets the payload
buffer.
- Added an output_limit sandbox constraint.  If the payload buffer
exceeds this amount the sandbox will be terminated.
- Consolidated the constraint checks in the Go API
    - removed the Memory(stat) and Instructions(stat) methods
    - added Usage(type, stat) to access all combinations