Fix optSemi type in Parser.y

The definition of 'optSemi' claimed it had type

([Located a],Bool)

Note that its production actually returns ([Located Token],Bool):

  : ';' { ([$1],True) }   -- $1 :: Located Token

Due to an infelicity in the implementation of 'happy -c', it effectively
resulted in 'unsafeCoerce :: Token -> a'.
See https://github.com/simonmar/happy/pull/134

If any consumer of 'optSemi' tried to instantiate 'a' to something not
representationally equal to 'Token', they would experience a segfault.

In addition to that, this definition made it impossible to compile Parser.y
without the -c flag (as it's reliant on this bug to cast 'Token' to 'forall
a. a').