Docs: add dates to manual and stdlib
Docs: cleanup dates chapter. Closes #8032

- Remove reference to Dates tests
- Use Sphinx :mod: reference to Dates module
Make date ranges internal guess() type stable to fix 32-bit testing failure
Remove some redundant dates tests to make dates test suite run a little faster.
Fix remaining 32-bit test failure by making sure Periods are parsed as Int64, regardless of WORD_SIZE
Dates manual chapter cleanup/update
Remove subtype of AbstractTime for Slot.
Cleanup some Dates module qualifiers
Put Dates tests in their own module to prevent method binding overwriting
Merge branch 'master' of https://github.com/JuliaLang/julia