Add fully open-ended range sub tree support

[werner77]

Right now half open ranges and closed ranges are supported, but there is no support for getting a fully open range:

(start, end)

or a half open range where the half-open boundary is on the lower end:

(start, end]

I suggest to add methods.

suffix(after: ...)

to BTree (and SortedSet where I need it).

I can help with a MR if wanted.