Fix infix:<before>/infix:<after> for Complex

Currently before/after uses `cmp` that's pretty liberal with allowed types,
which makes comparisons like `i after 42` "work". Per TimToady++[^1], these
should use infix:«<=>», which this patch makes it do.

However, one side effect of this is that <42+42i> after <41+42i> used to work,
but now throws, as infix:«<=>» only works with complex if the imaginary part
is negligible compared to the real part. So.... is before/after on two
Complex supposed to use `cmp` instead of `<=>`? I've no idea...

[1] https://irclog.perlgeek.de/perl6/2017-02-02#i_14028605

src/core/Numeric.pm

@@ -39,6 +39,9 @@ multi sub infix:<eqv>(Numeric:D \a, Numeric:D \b) {
 
 ## arithmetic operators
 
+multi sub infix:<before>(Numeric \a, Numeric \b) { (a <=> b) < 0 }
+multi sub infix:<after> (Numeric \a, Numeric \b) { (a <=> b) > 0 }
+
 proto sub prefix:<+>($?) is pure { * }
 multi sub prefix:<+>(\a) { a.Numeric }