Summary:
Adds a high performance source map builder that has certain restrictions compared to the `'source-map'` package:
- mappings have to be in the order of the generated source
- source files have to be started/ended separately on the generator. That means building up mappings is optimized for blocks of mappings that all belong to the same source file (or no file)
The implementation avoids allocation of complex value, i.e. strings and objects as much as possible by preallocating a buffer and using numeric character values throughout. The buffer is converted to a string only at the end.
This implementation is ~5✕ faster than using `'source-map'`.
Reviewed By: jeanlauliac
Differential Revision: D4392260
fbshipit-source-id: 406381302d951b919243a2b15e8bb75981e9f979
7ca5316562