Demo behaves poorly when `src/lib.rs` is deleted before a rebuild

[pnkfelix]

Due to rust-lang/cargo#4468, the demo here is less than satisfying, since cargo sometimes fails to run the build.rs script when it should.

Its possible that this could be fixed indirectly when we have support for rerun-if-changed (see pnkfelix/tango#12 )

But it would probably be best (since I do not expect rust-lang/cargo#4468 to be addressed with high-priority), to just change the demo to use a lib.rs file, and (maybe) change the tango documentation to encourage users to still use a lib.rs for their project root.

(Hopefully the root cause of rust-lang/cargo#4468 is not something that interferes with tango's basic model...)

[pnkfelix]

Its possible that this could be fixed indirectly when we have support for rerun-if-changed (see pnkfelix/tango#12 )

Evidence for this assertion is provided at: rust-lang/cargo#4468 (comment)