redundant_closure should be in the Perf group

[asomers]

The redundant_closure lint should be in the Perf group. All those closures take code space. The optimizer can't combine closures, even if they're identical. (Perhaps it can for Option::map; I haven't checked. But definitely for Future::map it cannot).

> cargo +nightly clippy -V
clippy 0.0.212 (481f7880 2018-10-30)

[Manishearth]

In most situations they get inlined so it's really about giving the optimizer more work.

[asomers]

In the case of Future::map they seem to get inlined, but multiple identical closures don't get combined. Perhaps it's because many of the Future::map invocations get inlined too? I don't know. But I do know that eliminating those redundant closures reduces the build size, even with LTO enabled.