Add lexing, parsing and some semantic checking for constant buffers.

test/parse/field_assignment.golden

@@ -2,4 +2,5 @@
  (fun vertex vs_main -> float4 (@position)
   [(:= pos float4 (@position))] 
   (:= x float 5) 
-  (= x 7)))
+  (= x 7) 
+  (return pos)))

test/parse/function_with_int_return.golden

@@ -1,3 +1,4 @@
 (program	
  (fun vertex vs_main -> int
-  [(:= pos float3)]))
+  [(:= pos float3)] 
+  (return 0)))