Cache source map results

[LarsDenBakker]

When repeatedly calling this library, it looks up the source maps each time. It would be more efficient if the source map information is cached between invocations.

I can work on a PR for that if this is something you are ok with adding.

[bcoe]

this seems like it would be a reasonable improvement, I'd be curious to see a benchmark that indicates it's worth doing though, as it introduces some complexity. For instance, annoying edge-cases like colliding source map names.

[SimenB]

also how it'd behave if Jest (or something else) is running in watch mode, how would we avoid stale caches?

[LarsDenBakker]

You can allow the calling library to provide the cache, which would be fresh for each test run.