This adds xfuncname and word_regex patterns for Rust, a quite
popular programming language. It also includes test cases for the
xfuncname regex (t4018) and updated documentation.
The word_regex pattern finds identifiers, integers, floats and
operators, according to the Rust Reference Book.
Cc: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>