feat: Implement enters operator >>

Also added paranthesization of fostr expressions.
  Finally managed to cast code generation in terms
  of bottomup processing of a local strategy.

  Resolves #1.

syntax/fostr.sdf3

@@ -18,10 +18,13 @@ context-free syntax
   Ex.Stdio    = <stdio>
   Ex.Sum      = {Ex "+"}+
   Ex.Receives = [[Ex] << [Ex]] {left}
+  Ex.Enters   = [[Ex] >> [Ex]] {left}
+  Ex          = <(<Ex>)> {bracket}
 
 context-free priorities
 
-  Ex.Sum
+  Ex.Enters
+  > Ex.Sum
   > Ex.Receives,
 
   // prevent cycle: no singletons