Implement new algorithm for local label processing

This change allows qualified references to local labels from other
sections: you can do `#send section.local` and it will take you to the
appropriate `:.local` label. When `section.local` is ambiguous due to
sections/locals sharing the same names, the reference resolves to the
local in the first such section.

tests/labels/diagnostics.txt

@@ -23,7 +23,11 @@
 :@
 :.local
 ---
-@Locals with section names
+@Local label definitions with section names
+'these are okay
 :touch
-#touch.skip
 :.skip
+#touch.skip
+'
+'this is not
+:touch.skip