Support JS backend

[flyx]

Nim's JavaScript backend is currently not supported because it doesn't support first-class iterators. Since it is unlikely that this will change in the near future, to support the backend, fastparse.nim must be refactored to not use first-class iterators.

Required changes:

• [x] Refactor parser to not use first class iterators
• [x] Refactor serialization to not use first class iterators
• [x] Find a way to deal with missing exception handling for JS target
• [x] Find a way to deal with missing StringStream for JS target (parsing)
• [x] Find a way to deal with missing StringStream for JS target (dumper)

What should be done to declare NimYAML fit for JS:

• [ ] All parser tests shall be green in JS
• [ ] The majority of serialization tests shall be green in JS, minus some that have issues depending on Nim's JS support
• [ ] Travis should run JS tests
• [ ] nimyaml.org testing ground shall be built in pure JS

[flyx]

~~Blocked by nim-lang/Nim#4635~~

has been fixed

[flyx]

Blocked by nim-lang/Nim#4802

workaround in place. work on this continues.