Convert perf to SPS per-CPU instead of per-thread.

Symbolication isn't quite right.
Resolve mmaps in perf-to-sps, and give up on multi-space symbolicate (for now).
Add symbolication to perf-to-sps

TODO: use a symbolication table to compact all these long names....
Clean up the perf data a little
Use a symbolication table to "minify" the perf JSON
Change fake symbol names from base-16 to base-92 encoding.

This takes another 12% off the uncompressed JSON size; it's unclear that
this actually matters.