[CM] Share the define-macro implementation of typed-define

[bil at ccrma.Stanford.EDU]

Thanks very much for the macro!
I'm not sure whether to include it in the s7
tarball.  In CL's "the" special form, the type
precedes the value (as in your macro), but in
typed-let it follows the value. I was thinking
that the type is optional in scheme,
so it should not be required.  Having two
ways to do it seems like it will invite confusion.