Add methods first_before and first_after.

[progval]

No description provided.

[progval]

ping?

[chaimleib]

I like the idea, but I want to allow for a strict option before I merge it, like this:

assert t.first_after(4, strict=false) == Interval(4, 7, '[4,7)')
assert t.first_after(4, strict=true) == Interval(5, 9, '[5,9)')

[chaimleib]

I'm also unsure what the best behavior is for if the returned interval could be one of several options with equal nearest bounds.