Add deprecated attribute.

[luqmana]

#723 - Add a deprecated items check to lint.

#[deprecated]
fn foo() {
    ...
}

#[deprecated = "use blah instead"]
fn bar() {
    ...
}

[catamorphism]

Can you include a test case (or two) testing what the attribute you've added? Thanks!

[catamorphism]

Merged, thanks!

[catamorphism]

Sorry, I had to back this out. It's not quite doing the right thing: you want to be checking for uses of deprecated items. Also, the test case didn't have an error pattern (look for //~ ERROR in existing compile-fail tests for examples of that). My fault for not looking at this more carefully. Feel free to submit an improved patch (or ask for advice on IRC).