Add ide-assist for redundant closure clippy lint

[joshka]

https://rust-lang.github.io/rust-clippy/master/index.html#/redundant_closure

Example:

xs.map(|x| foo(x))

Replace with:

// where `foo(_)` is a plain function that takes the exact argument type of `x`.
xs.map(foo)

[roife]

As a workaround, you can also set rust-analyzer.check.command to clippy, so that r-a will use clippy for flycheck, and you'll be able to directly use auto-fixes from clippy.