timing: add envvars for subsystem enable and verbose metadata
This commit adds the following environment variables:
- `JULIA_TIMING_SUBSYSTEMS` which can be set to, e.g.,
"+INFERENCE,-GC,METHOD_MATCH" to enable INFERENCE
and METHOD_MATCH and disable GC.
- `JULIA_TIMING_METADATA_PRINT_LIMIT` which defaults
to 10 and determines how many metadata items to add to
a single timing zone before truncating
This commit also includes other miscellaneous changes to incorporate
review feedback.
