mem: add &'a File support, unix_ext::FileExt support, other File fns
mem: add &'a File support, unix_ext::FileExt support, other File fns

Spoiler alert, but the rest of std::fs::File's functions will imminently
have support. This adds the rest of those functions. This requires the
use of a new FileCursor struct, as well as some other additions to weave
through the FileSystem code.

twmb/rsfs