Added docblocks for remaining core expectations
Corrected _proxy-to to accept variable shift length
Converted `be' tests into direct expectation handler test

The original implementation was not an expectation handler (for some
reason), so this was an artifact of that approach.
`output' parsing refactoring to allow sharing logic

This will be needed for match and possibly other output-driven expectation
handlers in the future.
`match' handler now defined in terms of output abstraction

Consequently, now supports `on' clause.
Extracted output expectations into separate source file
Output processing generalized