Introduce Attribute will lazy initializer, jnthn++

Inspired by rjbs++, this will actually first wrap the Attribute composer,
and in there, wrap the accessor with code that will call the indicated
block once to set the value, then call the original accessor again.

src/core/traits.pm

@@ -498,7 +498,28 @@ multi sub trait_mod:<handles>(Method:D $m, &thunk) {
 }
 
 proto sub trait_mod:<will>(|) { * }
-multi sub trait_mod:<will>(Attribute $attr, Block :$build!) {
+multi sub trait_mod:<will>(Attribute:D $attr, |c ) {
+    X::Comp::Trait::Unknown.new(
+      file       => $?FILE,
+      line       => $?LINE,
+      type       => 'will',
+      subtype    => c.hash.keys[0],
+      declaring  => ' attribute',
+      highexpect => <lazy>,
+    ).throw;
+}
+multi sub trait_mod:<will>(Attribute:D $attr, $block, :$lazy!) {
+    $attr does role {
+        method compose(|) {
+            callsame();
+            $attr.package.^method_table{$attr.name.substr(2)}.wrap(-> \self {
+                once $attr.set_value(self, $block());
+                callsame();
+            });
+        }
+    }
+}
+multi sub trait_mod:<will>(Attribute $attr, Block :$build!) {  # internal usage
     $attr.set_build($build)
 }