Sebastian Graf authored Sep 07, 2020 and Marge Bot committed Sep 12, 2020

`DsM` was previously defined in `GHC.Tc.Types`, along with `TcM`. But
`GHC.Tc.Types` is in the set of transitive dependencies of `GHC.Parser`,
a set which we aim to minimise. Test case `CountParserDeps` checks for
that.

Having `DsM` in that set means the parser also depends on the innards of
the pattern-match checker in `GHC.HsToCore.PmCheck.Types`, which is the
reason we have that module in the first place.

In the previous commit, we represented the `TyState` by an `InertSet`,
but that pulls the constraint solver as well as 250 more modules into
the set of dependencies, triggering failure of `CountParserDeps`.
Clearly, we want to evolve the pattern-match checker (and the desugarer)
without being concerned by this test, so this patch includes a small
refactor that puts `DsM` into its own module.