Have forked skim as Cargo dependency rather than as git submodule

[smheidrich]

Blocked by:

• cargo doesn't support "exeuctable dependencies" / "binary dependencies"
  • Tracking Issue for RFC 3028: Allow "artifact dependencies" on bin, cdylib, and staticlib crates rust-lang/cargo#9096
  • Support runtime executable dependencies. rust-lang/cargo#7971
• setuptools-rust doesn't support anything like that either