src/: Allow methods to have more than one entry each in a vtable.

src/: Allow methods to have more than one entry each in a vtable.

The entries are assigned distinct `roles' to distinguish them.  Each
role can have a different type.  To accommodate this a number of changes
are made to the API.  Note that no roles other than the standard `nil'
role are currently defined, so none of this change should have any
externally observable effect.

  * The `make-method-entry' method is replaced by `make-method-entries',
    which returns a list of entry objects.  The standard method on
    `compute-vtmsgs' collects these together into a big list.

  * Slots in the `vtmsgs' structure are now given names by the method
    entries directly, rather than being named after their messages.
    There is a new generic function `method-entry-slot-name' to make
    this work, and a little protocol `method-entry-slot-name-by-role' to
    make extending this machinery easy.

  * The `message-macro-name' function now takes a method-entry rather
    than a message, because each entry needs its own macro.

  * The `method-entry-function-name' function has grown an additional
    `role' argument.  The standard method inserts a non-nil role name in
    an unimaginative manner.

  * The standard method on `method-entry-function-type' now inspects the
    entry role, but its behaviour is unchanged except to check that the
    role is nil.