Profiling: Allow heap profiling to be controlled dynamically.

This patch exposes three new functions in `GHC.Profiling` which allow
heap profiling to be enabled and disabled dynamically.

1. startHeapProfTimer - Starts heap profiling with the given RTS options
2. stopHeapProfTimer  - Stops heap profiling
3. requestHeapCensus  - Perform a heap census on the next context
                        switch, regardless of whether the timer is enabled or not.