[plt-scheme] loop over environment variables and delete an environment variable

It seems that with the exception of using the FFI there currently is
no facility in PLT to either loop over the complete set of environment
variables or delete an environment variable. Is that a correct
conclusion? Is adding that sort of functionality in any plans?

My immediate use case is CGI/FastCGI where HTTP headers are sent to
the application as environment variables. It is not possible to know
the complete set of environment variable names a priori as the HTTP
request headers determine which variables will be present. Looping
over the complete set of environment variables is the way an
application can explore what is there.

Peter