Text.Lexer: improve `choice`
Text.Lexer: improve `choice`

Removed constraint requiring a non-empty list. Now, `choice` always
fails if the list is empty.

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