Change array syntax to prevent ambiguity introduced by RFC 439

This is an alternative to #498.

The change is not complex; most of the length of the RFC is due to my getting carried away when listing alternatives. I'm still open to changing details (e.g. `[x by N]` or `[x for N]` instead of `[N of x]`), but this is my current preference, and I feel that something along these lines is preferable to fixing this via a change to the RFC 439 range syntax.

[Rendered View](https://github.com/quantheory/rfcs/blob/master/text/0000-new-array-repeat-syntax.md)