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>

- Do not enforce (but assume) syntactic correctness of language
  constructs that go into hunk headers: we only want to ensure that
  the keywords actually are words and not just the initial part of
  some identifier.

- In the word regex, match numbers only when they begin with a digit,
  but then be liberal in what follows, assuming that the text that is
  matched is syntactially correct.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>