Generic iterator class for trees and variants

[hyanwong]

Description

Pull out the logic that iterates over trees and variants into a single base class, and allow this class to return a length.
Also allows left, right, copy to be passed to the trees() iterator (to match the variants() iterator), so that we can
iterate over a subset of the genome.

Also implements next() and prev() for trees, to match the variants iterator (they are handy anyway, right?)

Fixes #2625, #24

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[hyanwong]

BTW, this renders ts.aslist() a bit obsolete, as you can do ts.trees(copy=True), in the same way you do for variants.