…th repeated indices

When introducing the new broadcasting implementation (which came alongside greater
semantic guarantees with regards to aliasing source and destination), I found that
we needed a performance optimization for the common case where the destination was
`===` to an argument. This is quite common due to the `.op=` syntax and indeed is
safe in most cases as the iteration order between the destination and the source
is matched. This performance optimization is not safe, however, in the case that
the destination has multiple locations that refer to the same memory.

but still provide the hook for arrays to improve