Add TypeScript infrastructure and typings

[lundibundi]

• code is properly formatted (npm run fmt)
• tests are added/updated
• documentation is updated (npm run doc to regenerate documentation based on comments)
• description of changes is added under the Unreleased header in CHANGELOG.md

Since I always have the Iterator typings in my TS projects anyway I guess we could add them here, I'd support these to the best of my ability.
Not sure what would be the best way to go for other modules but since they rarely change I guess we could just type them as well. I would not change the process of PRing changes to include TS as people may not be familiar with it but rather someone updates them as we go forward (preferably before release 😄 ).

/cc @belochub

[lundibundi]

@tshemsedinov @nechaido @belochub @SemenchenkoVitaliy this is waiting for feedback (in general) to decide whether to go on with it. If we approve this idea/approach I could go on adding typings for other modules in common.
(I guess if you're fine with it you can just 👍 this comment, and otherwise comment)

[tshemsedinov]

I believe we need ts typings ASAP, tkanks for this initiative 👍 @lundibundi