This commit merges changes that were developed in the "racket7" repo.
See that repo (which is no longer modified) for a more fine-grained
change history.
The commit includes experimental support for running Racket on Chez
Scheme, but that "CS" variant is not built by default.
Changes signatures in `syntax/modcode` to accept `path-string?` arguments
instead of `path?`.
Before, the docs listed `path-string?` but the contracts used `path?`.
Now they agree.