Use GHC 8.0 as the minimum

[RyanGlScott]

This drops support for GHC 7.8 and 7.10, which allows us to clean up lots of code in the process.

Fixes #928. Fixes #964.

[phadej]

I run a dependency lower bound checker, and the lower bounds for following dependencies can be bumped.

bifunctors                     5.5.7
contravariant                  1.4 
distributive                   0.5.1 
exceptions                     0.8.2.1
hashable                       1.2.7.0
mtl                            2.2.1
parallel                       3.2.1.0
semigroupoids                  5.0.1
tagged                         0.8.6
template-haskell               2.11.1.0 
transformers                   0.5.0.0
transformers-compat            0.5.0.4 
vector                         0.12.1.2

I.e. solver was not able to find an install plan with lower versions for these dependencies (with any GHC>=8.0). I haven't checke whether we have CPP related to these versions, but it would help solver to be a slightly faster alone by pruning the search space earlier. (there is MIN_VERSION_mtl at least).

[RyanGlScott]

Thanks for checking! I'm a bit confused as to how the lower bound checker arrived at template-haskell-2.11.1.0, however, since template-haskell-2.11.0.0 is bundled with GHC 8.0.1.

[phadej]

Thanks for checking! I'm a bit confused as to how the lower bound checker arrived at template-haskell-2.11.1.0, however, since template-haskell-2.11.0.0 is bundled with GHC 8.0.1.

I used GHC-8.0.2. (And wouldn't mind if GHC-8.0.1 is excluded - I don't want to remember what bugs it had)

[RyanGlScott]

Very well. I'll update this to require GHC 8.0.2 (not 8.0.1) as the minimum soon.

[RyanGlScott]

I really wish I knew what is causing the mysterious CI timeouts, such as the one observed in this PR here. This isn't just limited to lens, as I've observed the same phenomenon in other repos here and here, among others.

[phadej]

@RyanGlScott yes, i have wondered whether it's haskell-ci fault or GHA flakyness. I don't really know where to ask for GHA related support though.

[RyanGlScott]

It's only one data point, but I haven't seen this happen in any CI jobs for base-compat, which doesn't use haskell-ci.

[RyanGlScott]

In any case, one of the GHC 8.0.2 builds did succeed, so I'm going to just merge this.