Refactoring to allow adding new structured types

Also, simplfy representation of zone types, using a simple struct.

R=bmeurer@chromium.org
BUG=
LOG=N

Review URL: https://codereview.chromium.org/225923002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20538 ce2b1a6d-e550-0410-aec6-3dcde31c8c00