Add Unboxed instances for Storable vectors

[ruifengx]

This is a follow up for the DoNotUnbox* family of newtype DerivingVia wrappers (#508). It allows treating Storable vectors as unboxed vectors. My personal use case is combining parallel arrays received from C FFI into a single Vector. I left a comment about this in #503, and did not get a response, but I still think this is a nice thing to have in vector.

I followed the implementation for UnboxViaPrim and named this new wrapper UnboxViaStorable. While working on this PR, I noticed that the documentation for UnboxViaPrim named the vector for Foo [M]V_Int instead of [M]V_Foo, so I also corrected them.

As a minor detail, the previous PR imports Data.Vector.Strict qualified as S, so I cannot use S for Storable and had to use St. I can change it if there is a preferred naming scheme for qualified imports.

[ruifengx]

I see some doctests errors and tried to fix them. Unfortunately, I cannot run doctests locally because it reports a lot of "Could not find module" errors, so I did it based on the logs from GitHub CI. After fixing the obvious typos, I am somewhat stuck right now because I do not quite understand how the module grouping in doctests.hs file works (I see that only 8.10.7 and 9.0.2 are still failing).

[Shimuuar]

I think PR is fine in general. We have support for deriving unboxed instances using primitive vectors, variants of boxed vector. No reason to not to add storable into the mix.

I'll try to look why doctests fail on GHC 9.0,8.10

[Shimuuar]

As it turns out failure was genuine. V_UnboxViaStorable must be in scope for deriving to work. Doctests just happen to work. I added tests to check that deriving actually works

[ruifengx]

Thanks, I see the problem now. doctest "opens" the module before running (in the same way as :m *SomeModule in GHCi), making all internal definitions available, which presumably have made the tests pass for most compiler versions.