Export the data types `GiveGCStats`, `DoCostCentres`, `DoHeapProfiles`,
and `DoTrace`, as well as the type synonyms `Time` and `RtsNat`. The
above data types appear as fields in the `-Stats` data types in
`GHC.RTS.Flags`, but since they only have `Show` instances, it is
practically impossible to due anything useful with the above types
unless they are exported.

Reviewers: hvr, ekmett, austin, ezyang, bgamari

Reviewed By: bgamari

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1030

GHC Trac Issues: #9970