add conjectural infix [&foo] parsing

Variants such as [&.foo], [&(...)] also allowed.
Variants with $ sigil also admitted (but not @ or %;
I suppose those could be allowed if keyed 2-dimensionally).

STD.pm6

@@ -3233,6 +3233,8 @@ grammar P6 is STD {
             | :dba('bracketed infix') '[' ~ ']' <infix=.infixish('[]')>
                                                     { $<O> = $<infix><O>; $<sym> = $<infix><sym>; }
                     [ <!before '='> { self.worry("Useless use of [] around infix op") unless $*IN_META; } ]?
+            | :dba('infixed function') <?before '[' <[&\$]> <twigil>? [<alpha>|'('] > '[' ~ ']' <infix=.variable>
+                                                    { $<O> = $<infix><O> // $¢.O(%additive); $<sym> = $<infix>; }
             | <infix=infix_circumfix_meta_operator> { $<O> = $<infix><O>; $<sym> = $<infix><sym>; }
             | <infix=infix_prefix_meta_operator>    { $<O> = $<infix><O>; $<sym> = $<infix><sym>; }
             | <infix>                               { $<O> = $<infix><O>; $<sym> = $<infix><sym>; }