Make extensive tests exhaustive if there are enough iterations available

[beetrees]

Currently the extensive tests will only check values within a functions domain (or that are non-NaN for multi-argument functions), even when there are enough iterations available to check every single possible value. This PR fixes that. There was also a bug where trying to exhaustively test fma/fmaf on 32-bit targets would immediately panic as the test case count doesn't fit in a 32-bit usize: this PR changes get_test_cases to use a separate u64 instead of ExactSizeIterator to fix this.

This also fixes #409.

[tgross35]

Thanks for all the fixes here, this looks great.

[tgross35]

Just as a note I'm currently planning to rework some of this so that the domain_logspace and extensive generators can use the same code, but that is pretty early on still #415.

Also if you're currently running this locally you need to skip the jn* and yn* tests, MPFR often throws an allocation failure that I haven't yet looked into.