Improve smartmatching with junctions

The last commit breaks smartmatching for junctions. While the compiler
could handle their threading when smartmatching, this would break any
existing code that has ACCEPTS candidates for junctions. Giving it a
default ACCEPTS candidate allows existing code to work again.

While not strictly necessary for this to work, this also moves handling
of junction smartmatching from Any to Mu. This allows `any(Mu, Any) ~~
Mu` to return `True` instead of `False`.

src/core.c/Any.pm6

@@ -15,13 +15,10 @@ my role  Numeric { ... }
 my class Any { # declared in BOOTSTRAP
     # my class Any is Mu
 
-    multi method ACCEPTS(Any:D: Mu:D \a) { self === a }
-    multi method ACCEPTS(Any:D: Mu:U $ --> False) { }
-
-    # use of Any on topic to force autothreading
-    # so that all(@foo) ~~ Type works as expected
-    multi method ACCEPTS(Any:U: Any \topic --> Bool:D) {
-        nqp::hllbool(nqp::istype(topic, self))
+    multi method ACCEPTS(Any:D: Mu:U --> False) { }
+    multi method ACCEPTS(Any:D: Mu:D \topic) {
+        # XXX: &[===] works with Any, not Mu!
+        self === topic
     }
 
     proto method EXISTS-KEY(|) is nodal {*}

src/core.c/Junction.pm6

@@ -176,12 +176,7 @@ my class Junction { # declared in BOOTSTRAP
         )
     }
 
-    multi method ACCEPTS(Junction:U: Mu:D \topic) {
-        nqp::hllbool(nqp::istype(topic, Junction));
-    }
-    multi method ACCEPTS(Junction:U: Any \topic) {
-        nqp::hllbool(nqp::istype(topic, Junction));
-    }
+    multi method ACCEPTS(Junction:U: Junction:D --> True) { }
     multi method ACCEPTS(Junction:D: Mu \topic) {
         nqp::hllbool(
           nqp::stmts(
@@ -343,7 +338,6 @@ my class Junction { # declared in BOOTSTRAP
         my int $elems = nqp::elems(positionals);
         my int $i     = -1;
         while nqp::islt_i(++$i,$elems) {
-
             # Junctional positional argument?
             my Mu $arg := nqp::atpos(positionals, $i);
             if nqp::istype($arg,Junction) {

src/core.c/Mu.pm6

@@ -21,6 +21,15 @@ my class Mu { # declared in BOOTSTRAP
     multi method ACCEPTS(Mu:U: Mu \topic) {
         nqp::hllbool(nqp::istype(topic, self))
     }
+    # Typically, junctions shouldn't be typechecked literally. There are
+    # exceptions though, such as Junction in particular, so this probably
+    # shouldn't be handled by the compiler itself. Having a default ACCEPTS
+    # candidate to handle junctions allows them to get threaded as they should
+    # while preserving compatibility with existing code that has any ACCEPTS
+    # candidates for Mu or Junction.
+    multi method ACCEPTS(Mu:U \SELF: Junction:D \topic) is default {
+        topic.THREAD: { SELF.ACCEPTS: $_ }
+    }
 
     method WHERE() {
         nqp::p6box_i(nqp::where(self))