equality of two Sequences is true iff both a element-wise equal and have
the same `count`.

This works as long as one of the Sequences is finite, it will diverge if
both are equal and infinite. A `check` will fail if `debug` is enabled and
both `a.finite.is_no` and `b.infinite.is_no`.

This is defined only if T : property.equatable, it will result in a compile-time panic
if this is not the case.