Improve the ctags function regular expression.

Before it would only catch lines starting `fn` or `pub fn`.

Now it can cope with:

- attributes (e.g. `#[test] fn`)
- external functions (e.g. `extern fn`, `extern "C" fn`)
- unsafe functions (e.g. `unsafe fn`)

… and any correct combination of these
(e.g. `#[test] extern "C" unsafe fn`).