promote_op() improvements
Use common promote_op() based on one() for all Number types:
this fixes promote_op(+, ::Bool), which returned Int, and
promote_op(==, ::Complex, ::Complex), which returned Complex{Bool}.
Add fallback definitions returning Any so that broadcast() computes
the appropriate return type, instead of using promote_type() which
is often wrong.
This fixes broadcast() when the return type is different from the input
type. Add systematic tests for operators and their return types.
