mpz/mpq mismatch

[BrunoDutertre]

Here's another simple example:

(declare T (! sc (^ (mp_add 1/2 1/4) 3/4) type))

lfscc incorrectly reports that this does not type-check.

Looks like there are two bugs in there

• in expr.h: method isArithTerm() does not handle all the arithmetic operators

• in check.cpp there's a built-in assumption that any arithmetic term has type mpz but that's incorrect.
  The term could also have type mpq.

  Here's the relevant piece of code in check.cpp lines 453--454

      if (code->isArithTerm())
           progret = statMpz;