Introducing the "lookup-constants runtime system" :)

[Stephen Weeks]

We've just discovered another complexity with compile time constants.  Some
(like Posix_error_access) should be set according to the target and some (like
MLton_safe) should be set according to the host.  I wanted to make you aware of
this problem and let you know that we are thinking about it.  You may want to
hold off until we have a solution, which may involve changing the _prim syntax.