avoid creating intermediate objects

[hrldcpr]

As discussed in #74, using high-level methods in loops can create lots of wrapper objects, which can reduce performance.

An example of fixing this sort of thing is f58c25f.

• [ ] find places where this should be changed
• [ ] change them, ideally without breaking abstraction barriers too much

[hrldcpr]

…relatedly, is it possible to implement IntTree.minusRange() in an efficient way?

[hrldcpr]

see also #41