* a single function to set up all environment parameters.
* improve `getarg's treatment of default thunk
* Add an error display handler that doesn't show the context and instead
add a ,bt command to show it.
* Rename `make-command-{reader,evaluator}' -> `make-xrepl-{reader,evaluator}'
* Move the commented-out ,meta block to a better place
* Protect the prompt computation against errors, to avoid infinite
exception output if an exception is raised.
* Add ",switch ?" to query namespaces, and ",switch - <name>" to remove
one. Forbid resetting the default initial `*' namespace.
* Clarify that multiple arguments can be sent to ,stx and fix it to
display the current syntax when there are no arguments.
* Various minor typos and improvements.
* Restore the use-last-arguments-by-default functionality of ,rr
* Re-do argument reading to make it easier to have a default
argument (as in ,enter and ,edit).
