Proposal: Agressively feature gate parts of the code as a way to make compiling faster

Compiling nalgebra is usually quite slow, mostly because of it's extensive use of generics but also given how large it's code base is.

On the other side when using nalgebra for purposes like graphics math most of it's features are rarely used and only a small subset is usually needed. I guess for other purposes other similarly small parts of it are used.

Having most of the code except for the very basics disabled by default and adding several features that enable the parts needed for common use cases would surely speed up compiling to a great degree in those cases where only a small subset of nalgebra is needed.

Perhaps a crate to take as reference as it does something similar is [syn](https://docs.rs/syn/latest/syn/index.html).