Add facility for emitting heap profile marker

[hvr]

When analysing heap profile diagrams I often want correlate certain points in the program execution with the corresponding point in the heap profiling diagram.

Currently, I resort to causing flatlines by burning cpu cycles with some dummy calculations which don't cause too much GC activity. But this is just a crutch, and doesn't work too well for concurrent/parallel Haskell programs.

It would be helpful to have a GHC facility to emit custom marker "events" (maybe with an user-suppliable associated numeric identifier to differentiate between multiple markers) to the heap-profile which could then be directly looked up in the resulting heap profile diagram (rather than having to recognize geometric graph features in the diagram).

[simonmar]

already have a ticket for this: #3021