renames and file consolidation

[yshavit]

• rename interface -> api
• move contents of interface into api
• rename the individual selectors to prefix them with sel_

This just makes it a bit easier to see at a glance which files do what:

• mod.rs is just the mod declarations
• api.rs is everything public to other modules
• base.rs is the base classes for selectors
• sel_<foo> is a selector for <foo>