RAII auto_mpfr and autp_mpz

RAII auto_mpfr and autp_mpz

The following adds two RAII classes, one for mpz_t and one for mpfr_t
making object lifetime management easier.  Both formerly require
explicit initialization with {mpz,mpfr}_init and release with
{mpz,mpfr}_clear.

I've converted two example places (where lifetime is trivial).

	* system.h (class auto_mpz): New,
	* realmpfr.h (class auto_mpfr): Likewise.
	* fold-const-call.cc (do_mpfr_arg1): Use auto_mpfr.
	(do_mpfr_arg2): Likewise.
	* tree-ssa-loop-niter.cc (bound_difference): Use auto_mpz;