Massively reorganize and clean up the whole Rust codebase (#478)

* Massively reorganize and clean up the whole Rust codebase

* Additional changes during code review

editor/src/viewport_tools/tools/mod.rs

@@ -0,0 +1,12 @@
+pub mod crop;
+pub mod ellipse;
+pub mod eyedropper;
+pub mod fill;
+pub mod line;
+pub mod navigate;
+pub mod path;
+pub mod pen;
+pub mod rectangle;
+pub mod select;
+pub mod shape;
+pub mod shared;