Major effort to plug slot-name leaks.

Arrange that all slot names, for structures and CLOS objects, are
internal symbols of the relevant package.

There used to be a number of bad words in slot names, including `class',
`method', `expr', `type', and `condition'.  All of these have gone.

I've used two main approaches.

  * Renaming the slots with a leading `%'.  For structures, this
    involves hacking the constructor function to initialize the slot
    from a dummy argument with a less unpleasant name, and setting up
    trivial reader and writer function wrappers, so there's a new macro
    `define-access-wrapper' in utilities.lisp to do this.  For CLOS
    objects,  the accessor functions are named explicitly so that's not
    a problem, but there's a lot of work needed to track down direct
    slot accesses through `slot-value' and `with-slots'.

  * For classes defined through `definst', I've instead named the slots
    with gensyms (at read time), because their names are used as part of
    automagically defined methods.

I may not have fixed everything: this is a rather invasive change.