Add Json stream wrappers

[satabin]

Closes #48

[ybasket]

asArray makes me think of [e1, e2, ...], not {"a":true, "s":[e1, e2, ...]} and I believe both have their use cases (though the first has a way simpler implementation, a pipe here could be user-friendly). I'm also thinking of deeply nested structures which now you can build by using several of these pipe, but a selector-based method like inject[F[_]](wrapper: Stream[F, Token], at: Selector, early: Boolean = true): Pipe[F, Token, Token] might be more user-friendly.

All these can be implemented in future PRs, I just want mention so that you can reconsider naming. For that, I'll approve, but not merge yet, do you as think. Definitely a very nice PR with awesome documentation, really beautiful!

[satabin]

Actually the name is wrap.asArray(at = "key") which is quite explicit I think. I miss the SmallTalk syntax for this kind of DSL. I was thinking about adding a asTopLevelArray as well, I can do it. Potentially rename asArray into something like asArrayInObject but I like it less.

I though about the selector and rejected it because it's hard to make it work properly in a streaming fashion and a user friendly way. Think about how you would put the stream into

{
  "a": {
    "b": {
      "c": true
   },
  "d": 1
}

at selector .a.b.c.d with early = true.

Arbitrary depth is just only corner cases, or then putting constraints into the doc and hoping users will keep it in mind at all time.

[satabin]

And on top of that, selectors allow for .a.[].b, how to interpret it? We would need JSON Path instead of selector here (the later are intended to be used to enumerate and select potentially several values, not identifying a unique one). This is something I thought about as well, but I think it's outside of the scope of this PR.