The `scheme/base' module had become unreachable from the `mred' module.
While that normally would be a good thing, it lead to troublesome
multiple instantiations of `scheme/base' that caused problems for
attaching further modules to the namespace.
Moved mred to private/mred and created a new mred that just re-exports the
definitions from the private module. This change is intended to allow for
contracts to be added later to where the definitions are re-exported.
