6 - add hash tables for symbols (useful for long programs)
7 - 0 is pointer - fix type compare
8 - add message if external function or variable not found.
9 - add float/double support (should be as small as possible while being
10 usable for RISC code generator too).
14 - fix preprocessor symbol redefinition
15 - better constant opt (&&, ||, ?:)
16 - function pointers to forward reference (patch code generator)
17 - add PowerPC code generator.
18 - add portable byte code generator and interpreter.