Annotate heap profile at certain times

[nomeata]

Just an idea I’d like to note down: When looking at a heap profile, I often have to guess what phase is what. It would be useful if there is a

GHC.Exts.annotateHeapProfile :: String -> IO ()

function where a vertical line shows up in the heap profile every time the function is called, and is labeled by the given string.

[simonmar]

Please add your comments to #3021