A language can now introduce a `configure-runtime' submodule that
is `dynamic-require'd before the enclosing module.
This new submodule protocol provides a more general and
easier-to-understand way of configuring the run-time environment for
a module's language, as compared to the `module->language-info'
path (through a `get-info' function, via a 'configure-runtime value,
and finally loading the specified module).
The `module->language-info' path remains in place, and it is
checked after a `configure-runtime' submodule is run, since
that order is likely to be the most backward compatible.
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