In order to make these more interchangeable in more places, always
return `(impl Iterator, u64)`. This will facilitate using other
generators for extensive tests.
This also allows reusing the same generator logic between logspace tests
and extensive tests, so comes with a nice bit of cleanup.
Changes:
* Make the generator part of `CheckCtx` since a `Generator` and
`CheckCtx` are almost always passed together.
* Rename `domain_logspace` to `spaced` since this no longer only
operates within a domain and we may want to handle integer spacing.
* Domain is now calculated at runtime rather than using traits, which is
much easier to work with.
* With the above, domains for multidimensional functions are added.
* The extensive test generator code tests has been combined with the
domain_logspace generator code. With this, the domain tests have just
become a subset of extensive tests. These were renamed to "quickspace"
since, technically, the extensive tests are also "domain" or "domain
logspace" tests.
* Edge case generators now handle functions with multiple inputs.
* The test runners can be significantly cleaned up and deduplicated.
