Enhancements to `expt`

[jpellegrini]

• Don't use Scheme arithmentic where not needed in exponentiation by squaring (the exponent is known to be a fixnum).
• Fast path for exponent=2 (much much faster).
• Change name exact_exponent_expt -> fixnum_exponent_expt
• Move the bignum check to my_expt, so the exponent is always a fixnum when calling fixnum_exponent_expt.
• Since the exponent is always a fixnum in fixnum_exponent_expt, compare with MAKE_INT(0) instead of using zerop.

Results:

• for some integer exponents, expt is faster
• specifically, for exponent = 2, it is more than twice as fast