Add optional Serde support

[dtolnay]

In #rust today someone was struggling to serialize rpds::Vector (logs). It would be great to have Serde impls for these data structures that can be enabled by a serde cfg.

[orium]

In the next few days I will probably not be able to get any coding done, but once I'm I will work on it. (Meanwhile if someone wants to work on a PR I might be able to review/merge it.)

[idupree]

I started implementing this in https://github.com/idupree/rpds/commits/serde-impls -- does it look reasonable so far?

[dtolnay]

Looks good so far!

[orium]

Yup, looks good. I only have some cosmetic/organization suggestions but I will leave that to the PR.

[idupree]

Might I have your cosmetic suggestions now so that I don't have to redo cosmetic changes for every data structure?

[idupree]

(I could send it in a provisional pull request if it's easier to comment on that way)

[idupree]

Implemented all of them (Vector, List, Queue, Stack, RedBlackTreeSet, HashTrieSet, RedBlackTrieMap, HashTrieMap). Note in case you had pulled the initial commit series: I've git-rebased since then. Pull request: #2

[orium]

Closed by #4