replace array_impl macro with const generics

[niklasad1]

Fixes #463 (comment)

Note, this doesn't work for an empty array because of serde AFAIU

[jsdw]

LGTM! Since there doesn't appear to be a zero-length array of zero-tuple impl, It's a little harder to pass in 0 params. Is it worth impling the trait on an empty tuple as well just to cover that?

(also, the $len part of the tuple impl isn't used so I wonder if it's worth keeping around?)