userdiff: add built-in pattern for rust

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>

t/t4018/rust-struct

@@ -0,0 +1,5 @@
+#[derive(Debug)]
+pub(super) struct RIGHT<'a> {
+    name: &'a str,
+    age: ChangeMe,
+}