Merge pull request #9 from darinmorrison/master

Cleanup; simplification; etc.
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.