Encode `Long`s as JSON strings, to simplify the use of `JSON.parse` in javascript.
Added `Long` unit test
All tests pass with split JSON/Jawn parsing, except for failureTests under Jawn
All tests pass with split JSON/Jawn parsing, except for failureTests under Jawn. Loads faster
`Js.Number` now takes `Double`s, to better match the semantics of Javascript. This does not affect our ability to losslessly transmit `Long`s, which are now encoded as JSON strings instead of numbers