Parse 'is', 'match' differently for non-DT input

Parse 'is', 'match' differently for non-DT input

In SMT 2.6, Datatypes are being introduced and they come
with testers (indexed identifier of the form (_ is c)) and
match expressions. This lead to failures in UFIDL
benchmarks in SMT-LIB because they declare the function
'is'. This commit changes the parser s.t. it does not
consider 'is' and 'match' special tokens unless the theory
of datatypes is enabled.