restore broadcasting array±scalar operations

As recently discussed in #5810, the deprecation of broadcasting `+` and `-` for arrays and scalars, while theoretically removing some ambiguity for the case of matrix+scalar (though it is not clear that this has ever been a problem in practice), seems to cause more annoyance than it is worth, and will be a continual headache for users migrating from NumPy, Matlab, etc.

This patch restores those functions.