add assignment operator and unify memory pools

The unification of memory pools also fixed some memory leaks and
hopefully reduced the mallocs of identifier strings significantly by
giving them the same pool as the token stream.

example.script

@@ -1,7 +1,13 @@
 a := 1
-b := 1 - 2 * 2 + 5
-c := a + b * 2 * b
-d := a + 4 * b * a
+a = a + 1
+b := a - 2 * 3
+b = 2 + b * a
+c := -b
+
+//a := 1
+//b := 1 - 2 * 2 + 5
+//c := a + b * 2 * b
+//d := a + 4 * b * a
 
 /*x := 1
 y := 1