Add message capability to snap()
Add message capability to snap()

snap("checkpoint one") will now store that message with the period and will
show it in the report:

  $ perl6 -e 'use Telemetry; snapper; sleep .2; snap("checkpoint"); sleep .1'
Telemetry Report of Process #89112 (2019-04-16T10:18:37Z)
Number of Snapshots: 6
Initial/Final Size: 81664 / 82052 Kbytes
Total Time:           0.31 seconds
Total CPU Usage:      0.01 seconds
No supervisor thread has been running

wallclock  util%  max-rss
   105562   0.88      368
    99626   0.05        4
     3318   1.38       12
   100438   0.04
     1959   3.31        4
--------- ------ --------
   310903   0.36      388

tbrowder/rakudo,tbrowder/rakudo,nbrown/rakudo,tony-o/rakudo,softmoth/rakudo,salortiz/rakudo,labster/rakudo,MasterDuke17/rakudo,rakudo/rakudo,softmoth/rakudo,tony-o/rakudo,labster/rakudo,labster/rakudo,MasterDuke17/rakudo,nbrown/rakudo,labster/rakudo,nbrown/rakudo,nbrown/rakudo,rakudo/rakudo,nbrown/rakudo,MasterDuke17/rakudo,rakudo/rakudo,salortiz/rakudo,tony-o/rakudo,softmoth/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,salortiz/rakudo,rakudo/rakudo,tbrowder/rakudo,tbrowder/rakudo,rakudo/rakudo,nbrown/rakudo,labster/rakudo,labster/rakudo,tbrowder/rakudo,tony-o/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,salortiz/rakudo,rakudo/rakudo,salortiz/rakudo,tony-o/rakudo,tony-o/rakudo,salortiz/rakudo,softmoth/rakudo,softmoth/rakudo