GHC.Stats cleanup
GHC.Stats cleanup

This does two things:

* The `RtsTime` type wasn't exported, but it is used as the type of
  several record fields. Let's export it and give it some documentation.
* Neither `RTSStats` nor `GCDetails` have `Read` or `Show` instances,
  but `GCStats` does! Let's fix this discrepancy.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: goldfire, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3625

shlevy/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc