Add defun motions for rust-mode.

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
auto merge of #10873 : pradeep90/rust/rust-mode-changes, r=pcwalton

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