Add support for function pointers.

Both uniform and varying function pointers are supported; when a function
is called through a varying function pointer, each unique function pointer
value across the running program instances is called once for the set of
active program instances that want to call it.

lex.ll

@@ -110,6 +110,7 @@ int16 { return TOKEN_INT16; }
 int32 { return TOKEN_INT; }
 int64 { return TOKEN_INT64; }
 launch { return TOKEN_LAUNCH; }
+NULL { return TOKEN_NULL; }
 print { return TOKEN_PRINT; }
 reference { return TOKEN_REFERENCE; }
 return { return TOKEN_RETURN; }