Impl traits for specific array lengths above 32

[withoutboats]

@kornel

There's work on fully generic constants in types, which would make it work for all array sizes:

I know This is intended as a stopgap.

[lang-team-minutes] Const generics

On Thursday at the language team meeting we discussed const generics, both in terms of RFC 1931 and the work that eddyb has done and plans to do on the const system. First, the tl;dr version: We believe that we can have an RFC accepted and const generics available on nightly by the end of 2017. We discussed the major outstanding issues, which have to do with the theory of equality we use in type checking. I’ll summarize the discussio below. One thing we want to throw to the community r…