Preserve parens when parsing list enumeration arguments.
Preserve parens when parsing list enumeration arguments.

List enumeration arguments are parsed as expressions and
then converted to types afterward. Previously parenthesis
productions were dropped; e.g. "[1..(2+2)*(3+3)]" was
parsed as "[1..2+2*3+3]" before resolving infix precedence.
This is now fixed.

Fixes #379.

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol