Move file-related functions out of instrument.
Return instrumented source from instrument.

This is mostly for debugging purposes.
Cleanups in instrument.

Move metadata-propagating code out of wrap methods, clean up whitespace.
Special case defn to preserve function names.
QA: fully spell instrd, remove dead code
Fix variable name in tests.
Add comment explaining why we special case defn
Merge pull request #8 from JacekLach/instrument-cleanups

Instrument cleanups