Improve performance of st.domains()

[tybug]

Closes #4201. My benchmark results:

Master

Field     Time:    Compared to: Time difference:
Baseline  0.000182 N/A
Text      0.353401 baseline     0.353219
Domains   3.109082 baseline     3.108901
URLs      3.952818 domains      0.843736
Emails    3.208230 domains      0.099147
Fake      0.065587 domains      -3.043495

This PR

Field     Time:    Compared to: Time difference:
Baseline  0.000170 N/A
Text      0.363252 baseline     0.363082
Domains   1.905849 baseline     1.905679
URLs      2.744583 domains      0.838734
Emails    2.032297 domains      0.126449
Fake      0.067094 domains      -1.838755

[tybug]

The previous flatmap dynamically created a lot of uncacheable strategies. st.tuples isn't enormously cacheable in general either, but is decently so when the length is the only thing that can vary.

            .flatmap(
                lambda tld: st.tuples(
                    *(st.sampled_from([c.lower(), c.upper()]) for c in tld)
                ).map("".join)
            )

Most of the performance gain was in moving the strategy definitions to __init__, though changing the flatmap here did have some additional effect as well.

[Zac-HD]

Thanks, @tybug!