Support for upcoming param_attrs feature

[phansch]

param_attrs is likely to be stabilized in Rust 1.39

The param_attrs feature is going to allow lint attributes on function, method and closure parameters:

fn foo(
    a: i32,
    #[allow(unused_variables)] b: i32,
) {}

We should review how this feature ties in with our existing lints. Apparently it did not cause any toolstate breakage when it was merged.

Specifically, we should create a list of lints that could benefit from parameter attributes and then update those lints to make use of them (if that's even required).