Attempted to move section over to the section-specific new breadcrumbs function.

I think the problem is, the index context needs to be a higher priority than the new context element, but when referencing data with `{.}` the new context element needs to take priority. I could either combine entries in the context tree so that variables like $idx and $len live side-by-side with real context elements, or try to implement a way to skip over index context elements when walking up the tree.

js/test_cases/explicit_context_setting/main.dust

@@ -327,6 +327,22 @@ Section vs Partial priority{~n}
 {/some_global}
 {>priority:explicit pet="snake"/}
 
+Section vs Exists priority{~n}
+=========================={~n}
+{?some_global:explicit pet="snake"}
+    {pet}{~n}
+{/some_global}
+{>priority:explicit pet="snake"/}
+
+Section vs NotExists priority{~n}
+=========================={~n}
+{^some_global:explicit pet="snake"}
+    MAIN {pet}{~n}
+{:else}
+    ELSE {pet}{~n}
+{/some_global}
+{>priority:explicit pet="snake"/}
+
 Section vs Partial priority Failed Walk{~n}
 ======================================={~n}
 {#doesnotexist:explicit pet="snake"}