Add new balance edge algos & reversible ReCom

[pjrule]

This PR is too big, which probably means the balance edge algorithms should live in their own file so that they can be reviewed separately. Right now, this PR contains:

• A contraction-based balance edge algorithm ported from GerryChain by @matthewkol186
• A memoization-based balance edge algorithm created by me
• An implementation of reversible ReCom
• Iterator-based APIs for ReCom and reversible ReCom
• API changes to make various components of the chain (balance edge function, tree function, etc.) more hot-swappable

[bsuwal]

I agree that this PR is too big. I would like the PR to be split further and with unit tests to feel better about it. One way you could break this PR down could be;

• I like the idea of a separate file, something like tree_partition.jl that can handle balanced cut algorithms.
• A single reversible recom PR without using the resumable library
• Adding the @resumable / @yield functionality separately. or using the Base.iterator suggestion that @matthewkol186 made last week?

I do understand that this seems like a lot of work. Let's talk about this though! Also thanks for the good function documentation!

[pjrule]

Let's split things into three files:

• recom.jl - regular ReCom proposal and auxiliary functions.
• revrecom.jl - reversible ReCom proposal and auxiliary functions.
• balance.jl - algorithms for finding balanced cuts (common to both chains)

@bsuwal suggests splitting the balanced cuts code out into a separate PR.