Improve documentation for immutable vectors

[konsumlamm]

This PR imrpoves the doc comments for the immutable vector related modules (and a small change in README.md/vector.cabal).

I'm sorry that this will probably take a while to review, the main reason for that is that all the doc comments are duplicated. The changes in Data.Vector, Data.Vector.Primitive, Data.Vector.Storable and Data.Vector.Unboxed are mostly the same as in Data.Vector.Generic.

I plan to make followup PRs for the documentation of mutable vectors and fusion.

[Shimuuar]

I only skimmed generic module. So far PR looks good

[lehins]

This looks great!!!! Thank you for doing all the tedious work.

The only thing that bothers me is the default (Int) for Primitive, Storable and Unboxed. I mention this in one of the comments, but I'll say it here too. Problem is that it is not visible to the user and I believe it will raise more questions that the problems it will solve.

[konsumlamm]

I'm not quite sure how internal/GenUnboxTuple.hs is supposed to work, so I just added the dots in there and in internal/unbox-tuple-instances.

[lehins]

@konsumlamm Could you also rebase on master, there are some conflicts since #397 has been merged

[Shimuuar]

I'm not quite sure how internal/GenUnboxTuple.hs is supposed to work, so I just added the dots in there and in internal/unbox-tuple-instances.

Looks like it's supposed to generate tuple instances as text.

Great work! I don't have any more suggestions, @lehins caught everything

[lehins]

@konsumlamm thank you!