Convert exercise dependencies from core_kernel to base

[sshine]

Affected exercises:

• meetup uses Date from Core_kernel and can't simply be downgraded, I think.
• zipper uses nothing outside Base and can trivially be downgraded.
• word-count uses String.Map from Core_kernel and can somewhat simply be downgraded.
• nucleotide-count uses Char.Map from Core_kernel and can somewhat simply be downgraded.
• react uses the deprecated succ instead of Int.succ and Int.Table (which looks like a specialized Hashtbl) can be downgraded with a bit of fiddling.

The remaining 40 exercises all depend on Base.

[stevejb71]

Meetup could define its own date type.

[sshine]

Then the exercise would have to contain a function that converts a date to a weekday. I don't know if that's hard, but it's something I would like to expect is solved by a date library.

I would prefer if the exercise encouraged the use of existing popular libraries when ones exist.

I think this exercise belongs to Core_kernel unless there's another date module that is also popular.

[sshine]

I'll leave meetup as-is until a proposal to change it is made.