Adds Rewrite trait and basic impl for lists

The Rewrite trait is meant to abstract the common pattern of rewriting
some code within a given width and offset.

Author: cassiersg

src/lib.rs

@@ -58,6 +58,7 @@ mod lists;
 mod types;
 mod expr;
 mod imports;
+mod rewrite;
 
 const MIN_STRING: usize = 10;
 // When we get scoped annotations, we should have rustfmt::skip.