Emit source mappings during JS generation

Summary:
To avoid emitting the same mapping twice or emitting wrong mappings,
emit segments for the source map in specific cases of the `match`.
For now, we emit for every literal and identifier,
as well as specific keywords such as `return`, `if`, etc.

Reviewed By: tmikov

Differential Revision: D31172158

fbshipit-source-id: 3b7f5778ecc5dbe2e49b9dac7e41ee1100a9cc8c