Code cleanup to be more conforming to style guide.

- Return `Unit` instead of using procedure syntax.
- Explicitly listed return type for `def`s.
- Added/removed `()` for arity-0 methods depending on whether or not they have side-effects.
- Removed unused imports.
- Fixed missing "`" in doc strings.
- Removed redundant `final` qualifier.
- Renamed values to prevent shadowing.
I'm pretty sure Martin O. said the big reason for the empty-apply conversion was to accommodate the parens of Java.toString, which would create a world-wide shortage of parens otherwise. (As I recall from a SO answer.)