Remove largely unused context from Visitor.
Track the visited AST's lifetime throughout Visitor.
syntax: add a custom owned smart pointer in ptr::P.
syntax: ast: replace Gc<T> (previously @T) with P<T>.
syntax: fold: use move semantics for efficient folding.
syntax: ast_map: use borrowed references into the AST.
syntax: fix fallout from using ptr::P.
syntax: tests: fix fallout from using ptr::P.
rustc: fix fallout from using ptr::P.
rustdoc: fix fallout from using ptr::P.
fourcc: fix fallout from using ptr::P.
hexfloat: fix fallout from using ptr::P.
regex_macros: fix fallout from using ptr::P.
syntax: implement in-place folding of P<T> and Vec<T>.
syntax: document the ptr module.