Add Junction failover for named args in multis

This makes multiple dispatch failover also account for named arguments,
not just positional ones. This is not precisely the same semantics as on
the master branch: there, we invoke a candidate that would fail to bind
because of an encountered Junction arg, and rely on the single dispatch
binder failure to trigger auto-threading. This was only true of Junction
named arguments to multis, not of positional ones, and thus rather
inconsistent. (It was also certainly an implementation accident rather
than carefully considered semantics.) The new semantics do it as a bind
failover exclusively.

src/vm/moar/dispatchers.nqp

@@ -1796,14 +1796,15 @@ sub form-raku-capture($vm-capture) {
     $raku-capture
 }
 sub multi-junction-failover($capture) {
-    # Take a first pass to see if there's a Junction arg.
-    my int $num-args := nqp::captureposelems($capture);
+    # Take a first pass to see if there's a Junction arg (we look at both
+    # named and positional).
+    my int $num-args := nqp::dispatch('boot-syscall', 'capture-num-args', $capture);
     my int $i;
     my $found-junction;
     while $i < $num-args {
-        my int $got-prim := nqp::captureposprimspec($capture, $i);
+        my int $got-prim := nqp::dispatch('boot-syscall', 'capture-arg-prim-spec', $capture, $i);
         if $got-prim == 0 {
-            my $value := nqp::captureposarg($capture, $i);
+            my $value := nqp::dispatch('boot-syscall', 'capture-arg-value', $capture, $i);
             if nqp::isconcrete($value) && nqp::istype($value, Junction) {
                 $found-junction := 1;
                 last;
@@ -1813,12 +1814,12 @@ sub multi-junction-failover($capture) {
     }
 
     # If there is a Junction arg, then take another pass through to put type
-    # guards on all positional argument types.
+    # guards on all argument types.
     if $found-junction {
         $i := 0;
         while $i < $num-args {
-            if nqp::captureposprimspec($capture, $i) == 0 {
-                my $arg := nqp::captureposarg($capture, $i);
+            if nqp::dispatch('boot-syscall', 'capture-arg-prim-spec', $capture, $i) == 0 {
+                my $arg := nqp::dispatch('boot-syscall', 'capture-arg-value', $capture, $i);
                 my $tracked := nqp::dispatch('boot-syscall', 'dispatcher-track-arg',
                     $capture, $i);
                 if nqp::istype_nd($arg, Scalar) {