Sub-Module the functional-red-black-tree library about memdown

[vweevers]

The library hasn't been meaningfully updated in 6 years.

A testament to its quality 😉 I see no reason to believe that the project is dead (the open PRs are only related to TypeScript). If it is, I would prefer that someone takes over maintenance. In any case, I'm opposed to vendoring. Any improvements will also benefit the module by itself - which I've used in the past as a synchronous alternative to memdown.

Let's first just try PRs to functional-red-black-tree, for example for an upsert method.

[MeirionHughes]

okay fair enough. I spent some time trying to get into it last night; background reading on "purely functional" BST and trying to see how its done in the lib. need to do some more playing to fully understand the path-stacks though.

[MeirionHughes]

@vweevers yeah I see what you mean. its rock solid. Its performance is hard to beat - by a wide margin. thought I'd give a functional avl a go: I can't get close. even for basic find I'm 50% slower. :(