Add some doctests

[Shimuuar]

They're mostly copied from #101 with exception of maximum/minimum related ones (see #362, they're to be fixed separately). Also examples from accum & accumulate are converted to doctests

[lehins]

Taking a quick peek at it looks great. One comment I have right now is that it is important to stay consistent with respect to doctests because users will mimic them in their code. In this case in particular I am talking about qualified imports. I've personally almost always used these:

importqualifiedData.VectorasVimportqualifiedData.Vector.MutableasMVimportqualifiedData.Vector.GenericasVGimportqualifiedData.Vector.Generic.MutableasMVGimportqualifiedData.Vector.StorableasVSimportqualifiedData.Vector.Storable.MutableasMVSimportqualifiedData.Vector.PrimitiveasVPimportqualifiedData.Vector.Primitive.MutableasMVPimportqualifiedData.Vector.UnboxedasVUimportqualifiedData.Vector.Unboxed.MutableasMVU

Those were also the names I've used in recently added PR #360

I am not terribly set on those names, but keeping them consistent I think is very important. However I really don't like the as S for storable vector, because often import qualified Foreign.Storable as S also shows up in scope,

[Shimuuar]

I always used G/V/P/U for qualified imports. But VG/V/VP/VU is just as good to me. I'll update doctests

[lehins]

@Shimuuar If you are busy I can update those qualified imports. I'd, like to merge this PR, just so I can included these doctests in the vector-0.12.2.0 release I'd like to cut today

[Shimuuar]

@lehins Done.

[lehins]

@Shimuuar Nice, thank you. I'll merge it when CI completes