Replace various type M env ms with common intersection

[sjakobi]

The following files contain definitions of some type M env m which largely seem to be copies of each other:

src/Stack/Build.hs
src/Stack/Build/Execute.hs
src/Stack/Build/Installed.hs
src/Stack/Docker.hs
src/Stack/Nix.hs
src/Stack/SDist.hs

This leads to some redundancy and to less informative haddocks (example) because none of these types are exported.

I was wondering whether there would be a point in creating an alias of the intersection of these type constraints and exporting it (from Stack.Types.StackT maybe), so it shows in the haddocks. One idea for a name would be Common.

This idea conflicts with #1297.

[mgsloan]

Makes sense. I think the reasoning for having module local definitions of it is to make it easy to add a constraint to everything in the module. I also find it hard to follow, though.

I find the concrete approach to be the easiest to read. It provides better type errors, and better performance (nothing that really matters, though). It does sacrifice some flexibility, but I think in most cases we aren't using that flexibility. Then again, perhaps this is an aspect of stack's design that we don't want to sacrifice. Thoughts, @snoyberg @chrisdone ?

[sjakobi]

I'm rather leaning towards the approach proposed in #1297 now.