Sets the wormhole entry code in a wormhole status object
General Form: (set-wormhole-entry-code whs code)
See wormhole. Whs should be a well-formed wormhole status and
code should be :ENTER or :SKIP. This function returns a new
status with the specified entry code but the same data as whs. It avoids
unnecessary consing if the entry code for whs is already set to
code. This function does not affect state or a wormhole's hidden status.
It just returns a (possibly) new status object suitable as the value of the
lambda expressions in wormhole-eval and wormhole.