float_pow: Put *all* of the burden on the libm pow in normal

float_pow:  Put *all* of the burden on the libm pow in normal
cases.
powu:  Deleted.

This started with a nonsensical error msg:

>>> x = -1.
>>> import sys
>>> x**(-sys.maxint-1L)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: negative number cannot be raised to a fractional power
>>>

The special-casing in float_pow was simply wrong in this case (there's
not even anything peculiar about these inputs), and I don't see any point
to it in *any* case:  a decent libm pow should have worst-case error under
1 ULP, so in particular should deliver the exact result whenever the exact
result is representable (else its error is at least 1 ULP).  Thus our
special fiddling for integral values "shouldn't" buy anything in accuracy,
and, to the contrary, repeated multiplication is less accurate than a
decent pow when the true result isn't exactly representable.  So just
letting pow() do its job here (we may not be able to trust libm x-platform
in exceptional cases, but these are normal cases).