Add impls for powers of 2 length arrays.

[richard-uk1]

It would be really nice if there could be more array impls. I know that const generics are coming, but this would allow me to avoid having to write a lot of these impls myself, which would save me a lot of time, and make the eventual conversion to const generics a zero-work job for me. :)

I'm just submitting this as a PR because the change is so small, it's the same amount of effort as writing an issue.

[rust-highfive]

r? @cramertj

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[alex]

See also #51318 :-)

[richard-uk1]

I'm tempted to close this since I'm happy to just start using const generics on nightly when they land.

Totally more excited about them than rust 2018 :)

[richard-uk1]

I've made a crate to implement the traits I need for newtypes (basically by copying the macro in the rust source). It's on crates.io. So for me I'm happy using this because it means I can continue to support stable, but I'll leave this open in case someone else wants, I don't need it any more though.

[cramertj]

cc @rust-lang/libs -- what's the current policy on extending these array impls?

[alexcrichton]

Historically the libs team has decided to not change what we have, although it's been awhile so we can revisit this decision during the next triage meeting as well.

[alexcrichton]

Ok the libs team discussed this yesterday and have reached the same conclusion as before, so I'm going to close this. Thanks regardless though for the PR @derekdreery !