126: Fix generic bounds on buffer mapping r=grovesNL a=kvark
Smaller (and incomplete!) alternative to #119 while it's still discussed.
One missing piece here is alignment. cc @Coder-256
Also, importing `wgpu-core` as just "core" wasn't the best idea 😅 : it collides with the actual `core`.
Co-authored-by: Dzmitry Malyshau <kvarkus@gmail.com>