Removed unnecessary constraints on TokenParsing instances.

the instrances for Unspaced m and Unhighlighted m both had MonadPlus m
constraints. These aren't necessary because Alternative (a superclass
of TokenParsing) provides empty, and the constructors themselves can
be used rather than 'lift'.
an Unlined parser transformer, that disables trailing newline consumption.
Merge pull request #12 from startling/parser-transformers

Some changes to the parser transformers.
Export `Unlined(..)` for #12