Finish InBody

Except for various cases I didn't feel like implementing yet :)
AfterBody and AfterAfterBody
Reduce Makefile duplication for examples

And rename them.
Fix catchall case for AfterHead
Add higher-level functions to drive the parser
Implement a simple refcounted DOM
Rename lib.rs -> bin.rs for executable crates
Remove some unneeded .into_string()

Following the demise of ~str.
Suppress remaining tree builder warnings
Add a helper for parsing a single String
Make rcdom::{Handle, WeakHandle} transparent aliases
Actually permanently change the tree builder mode when we reprocess
Don't crash when we split-ignore whitespace into nothingness
Add more comments for rustdoc
Implement Text mode (except for the hard part)
Give tree builder fail!()s unique messages
Add a helper for iterating over active formatting elements
Implement <a> in InBody
rcdom: Append text to existing nodes when they exist
Escape token descriptions in tree builder debug/error output