Format file sections

Use 3 semicolons, remove trailing periods.
Simplify indentation command

Remove redundant branches in `swift-indent--calculate-indentation`
retained from the original rust-mode implementation.

Fixes #13
Add docstrings for internal indent commands
Replace `if` exprs with `when` exprs

Fix degenerate `if` expressions with no else clause. Use with `when`
expressions instead.