Add support for heap profiling events

[maoe]

This MR adds support for the new heap profiling events support in eventlog.

There are some points that are worth mentioning:

• EventLogFormat.h is updated. As in #27, we don't use the upstream version so I manually applied relevant patches from GHC.
• I chose to use Text instead of String for various text fields in the heap profiling events. Also I switched over the textual serializers from bytestring's builder to text's.
• The textual format of heap profiling events is kind of arbitrary. I'm happy to change it if anyone has a better idea.
• The code format of heapProfParsers looks a bit different from the other parts of the module because I'm not comfortable with the current format which causes a lot of HLint warnings.. Please let me know if it's better to make it similar.

[maoe]

Does anyone volunteer to review this?

[maoe]

It's been a week. I think I'll just merge this if there are no objections.

[simonmar]

Took a brief look, no objections from me.

[maoe]

Thanks! I'll merge this and release v0.7.0 when I have time today.