Extend the test runner so that we can choose to run only a specific test file.
Refactor SourceMapConsumer and SourceMapGenerator to share the same internal
representation of a single mapping, and to share comparators of those mappings.
Add SourceMapConsumer.fromSourceMap to create SourceMapConsumer instances from
SourceMapGenerator instances.
Use for loops instead of .forEach for some perf gains
Merge pull request #78 from fitzgen/source-map-consumer-from-source-map

Add `SourceMapConsumer.fromSourceMap`