feat: Implement enters operator >> (#8)

Also adds parenthesization of fostr expressions.
  Recasts code generation in terms of bottomup processing
  of a local strategy.

  Resolves #1.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #8
Co-Authored-By: Glen Whitney <glen@nobody@nowhere.net>
Co-Committed-By: Glen Whitney <glen@nobody@nowhere.net>

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