Add memory usage logs

[mpickering]

As I suggested on IRC

9:15 AM  <mpickering> Has anyone actually looked into why hls "uses a lot of memory", seems like a lot of people complain about it 
9:15 AM <mpickering> Just knowing some RTS metrics, heap size and live bytes would be informative
9:16 AM <mpickering> Perhaps can build with `--with-rtsopts=-T` and then periodically dump some heap stats to the log
9:16 AM <mpickering> so if someone complains then there are numbers
9:17 AM <mpickering> You would expect heap size to be about 2 * live bytes
9:17 AM <mpickering> but I suspect that it isn't

Stats can be read from https://hackage.haskell.org/package/base-4.14.1.0/docs/GHC-Stats.html

Then a test could also be added to CI which described some desirable properties of memory usage over time. (Something like this test)