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.