Introduce representation types

Also:
- improve type pretty-printing,
- update doc comments,
- some renamings for consistency.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/176843006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20025 ce2b1a6d-e550-0410-aec6-3dcde31c8c00