844 New sequence functions: names

[ChristianGruen]

Closes #844

[michaelhkay]

I am having second thoughts about dropping items-starting-where, in particular (but also items-ending-where). I think items-starting-where probably has more use cases than items-after. Note also that items-after() can be simply derived as items-starting-where() => tail(), but it's less easy to derive items-starting-where(). I'm now inclined to keep these with the names subsequence-starting-where() and subsequence-ending-where().

[ChristianGruen]

I'm now inclined to keep these with the names subsequence-starting-where() and subsequence-ending-where().

…which really gets awkward, and we'd have array:subarray-starting-where.

I'd be very much in favor of resorting to fn:take-while and fn:drop-while/fn:skip-while, which are available in various other languages.