Text.Parser: refactor and generalize combinators
Text.Parser: refactor and generalize combinators

Renames combinators `optional` and `maybe` to `option` and `optional`, to be
more in line with Haskell parser combinator semantics.

kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev