Safe JSON prep

[seanbaxter]

We want to build a rich example with fully-safe JSON parsing. This can use reflection to (de)serialize structs.

• What are the language features we still need implement?
• What are the library components we need?

[cmazakas]

We're going to for sure need a safe numeric parser.

I'm not sure how to create a struct using reflective methods. How do we handle objects-of-objects? Do we construct an object then gradually relocate into the data members? Do we attempt placement new?

We're probably going to need a FromIterator trait for constructing arrays as well. I forget that a big part of JSON is arrays.

std2::vector will need an iterator-based constructor which I can work on adding in addition to a numeric parser of some kind. I think Matt Borland is the expert here.