libsyntax Parser does not allow parsing view items individually

[murarth]

Parser method parse_item will unconditionally fail if it encounters a view item (use or extern crate).
This is desired behavior if parsing the body of a function. However, there are no public methods that allow parsing either of these items individually. This makes it impossible to parse the full range of possible input to a REPL.
If it's acceptable, I could make a pull request for public methods parse_use (which already exists, privately) and parse_extern_crate.