Improve compilation time

I was wondering if it is possible to improve the compilation time for `vector`. On my machine, compiling it from scratch currently takes about 1.5 minutes (1 m 35 s), which I feel like is too long. For comparison, `containers` takes 45 seconds and `unordered-containers` takes 23 seconds. Yes, I know, cabal caches the build, but it's still annoying (you still have to compile it when switching to a new GHC version, contributing to `vector`, in CI, when non-Haskell people want to install a tool using `vector`,...).

Some ideas that might help to decrease compilation time:
* disable implicit fusion (#457)
* split `vector` into several smaller packages (e.g. `vector-core`, `vector-storable`, `vector-unboxed`), so that users only have to depend on the parts they actually use (`vector` would stay the same, so people could just keep using it)
* don't use `-O2` (find out which flags significantly improve performance and only enable those)
* reduce inlining (this is probably hard without also reducing performance)
* avoid redefinitions for specialization (this would be a breaking change, but idk how big the impact would be)

I'm curious how realistic you find my ideas and if you have other ideas to improve compilation time.