Improve documentation substantially. Add a message which is displayed

Improve documentation substantially.  Add a message which is displayed
at the start of the Scheme process to tell novices what to do.  This
can be disabled by setting it to the empty string.  Change the
handling of the keymaps so that they do not inherit.  Implement a
command to signal the interpreter to proceed from a breakpoint.
Change the names of the major modes to be more systematic.