Add defun motions for rust-mode.

[pradeep90]

Specifically, we can now use:

• beginning-of-defun
• end-of-defun
• mark-defun

where "defun" means a Rust item.

• Add tests in rust-mode-tests.el
• Fix indentation in rust-mode-tests.el
• Add support for trait to Imenu

[metajack]

Things I noticed:

• mark-defun has an extra line before the item included
• beginning-of-defun when outside a method takes you to the previous method instead of the beginning of the impl. also at after an impl block, it takes you to the last method. Is this the expected behavior?

[metajack]

r=me otherwise

[pradeep90]

Hi @metajack

mark-defun has an extra line before the item included

Yes, it is the expected behaviour. It works that way even in the default emacs-lisp-mode.

beginning-of-defun when outside a method takes you to the previous method instead of the beginning of the impl. also at after an impl block, it takes you to the last method. Is this the expected behavior?

Yes. It goes to the first item before point, not the previous item at the same level.

backward-up-list (C-M-u) generally works for going to the enclosing defun. So, we can use that in a separate function to do what you asked for if people really need it.