Improve sqrt handling of large integers and large and small rationals.

Improve sqrt handling of large integers and large and small rationals.

* libguile/numbers.c (exact_integer_is_perfect_square,
  exact_integer_floor_square_root): New static functions.

  (scm_sqrt): Use SCM_LIKELY.  Add 'scm_t_inum' variable in inum case to
  reduce the number of uses of SCM_I_INUM.  Rename 'mpz_t' variable.
  Remove unneeded sign check.  Handle bignums too large to fit in a
  double.  Handle fractions too large or too small to fit in a
  normalized double.

* test-suite/tests/numbers.test ("sqrt"): Add tests.