75b57ad8c0
This is being done in preparation for writing a format from rbspy to import into speedscope, whose internal file format is a list of stacks (111689fe13/src/storage/v1.rs (L13))
For now, speedscope will always export the evented format rather than the sampled format, but will accept either as input. I also added tests for existing versions of the file format to ensure I don't accidentally drop support for a past version of the file format.