This is along the same vein as #148, but less drastic and specialized.

See that there have already been thoughts in this direction here: https://github.com/mozilla/source-map/blob/master/lib/source-map/source-map-generator.js#L297-L299

1. Small gains from keeping mappings always-sorted as an invariant
2. Especially cheap insertion of mappings when added in-order

Implementation wise this could be implemented by using a custom data type for _mappings.

Some sort of heap, or even a plain array with sorted insertion and using push for mappings beyond maximum seen (line, column).