Migrate to flat workspace

[panstromek]

Currently, the workspace is a bit messy tree. We want to migrate to a simple flat structure, as described in https://matklad.github.io/2021/08/22/large-rust-workspaces.html

[panstromek]

Mostly done, but tests and webzebra are still separate. I am not sure what to do with webzebra, because it's separate project also with UI, but I'll probably move tests crate, I am just not sure where to put snapshot-test resources and if the paths will work correctly everywhere.