Simplify replicate; inline
Simplify; move string_utils to util::string
Move prepend into util::vec
Use ::new instead of ::tree
Renaming

In particular, using to_string is probably bad because of the std lib
function ToString::to_string(). However, we could implement an instance
of ToString for Doc.
Merge branch 'master' of github.com:darinmorrison/rust-pretty into darinmorrison-master
(Attempt to) Merge branch 'darinmorrison-master' into dlist

Conflicts:
	src/pretty/doc.rs