Add AsArrayRef/AsArrayMut traits

[tarcieri]

Impl'd for Array<T, U> and [T; N].

This is an alternative to #134, given the trouble we had in #131 with adding impls of core traits to [T; N] and the ensuing inference conflicts it can cause.

The safest thing we can do is define our own traits, and these traits bound on AssocArraySize so they're able to handle everything related to the array size themselves, making them generic around only T.

With this we can revert #134, which still has the potential to cause problems similar to #131. Using our own trait guarantees we won't break inference in existing code.

[tarcieri]

@newpavlov if this looks good to you, I'd like to merge this, revert #134 to prevent any future inference problems similar to #131, then cut a new major version and start updating... well, everything (including a new release of dbl)