Fix Cargo manifests

[marc0246]

A couple fixes to the Cargo manifests of our public crates:

• vulkano had both a macros and vulkano-macros feature, the latter of which is completely useless and can't be used. Now there's only macros so it's a bit less confusing hopefully.
• Each crate's manifest links to its subdirectory in the repo, instead of the repo as a whole.
• Each crate's manifest links to its own docs page, rather than vulkano's.
• Each crate now has the readme linked.
• Other miscellaneous improvements.

[Rua]

The new repository URLs are all deadlinks, so I think something went wrong there.

[marc0246]

Oof, I don't know how I didn't see that.