Direct implementation of getMappings for speed.

On a build with 70k files in the assembly, this takes the total cached
assembly time from ~50s to 20s.  The time due to `getMappings` goes from
~36s to ~4s.  From what I could tell, the bulk of the time was due to `IO.relativize`.  It is possible there is something lower-level like `IO.relativize` that should be further optimized, but this worked so well I stopped here.