- Minor changes to how hash values are computed for scrambling in the
ZSobolSampler.
- Folded util/bits.h into util/math.h.

Small cleanups in EqualAreaSquareToSphere():
- use std::sin/std::cos rather than approximations
- replace FlipSign() with with std::copysign

Mostly minor code motion and clang-format formatting.
RandomPermute4() is now gone, since it's no longer used, however...

No meaningful changes, though in FastExp(), underflow and overflow are
now handled more rigorously.

The rounded floating-point math intrinsics are used on the GPU now, which
gives tighter bounds thanks to less unnecessary conservative rounding.

No change to functionality, but the Interval class is no longer templated
on the Float type. This added a whole bunch of complexity that wasn't
actually being used.

Note: the Film's "scale" parameter has been removed, as it is redundant given "iso" now.

Note: some parameter values to the "spherical" camera have been renamed.

Fixes warnings that were noted in issue #20.