Fix framework doc typo
support libpng16
Clean up lam-result.

Make drest a list instead of a pair, replace an int-err with a match error
which has source location, and remove static name for rest argument in error
case.
Massive cleanup of requriements in TR.
Reduce code duplication in Redex enum
Added var-except
Minor change to Redex contracts to maintain bijection internally
Simplified redex enumeration internals