[DGD] object types

[Felix A. Croes]

Within the next few days, I hope to release an important new feature:
object types.  It is going to be possible to add typechecking to objects:

    /* normal object */
    object obj;

    /* objects that inherit /kernel/lib/user */
    object "/kernel/lib/user" user1, user2;

Object types can be used anywhere where you can normally use 'object':
for variables, function types, function parameters, and in casts.

    obj = user1;	/* always works because obj has no specific type */
    user1 = user2;	/* always works because user2 is of the same type */
    user2 = obj;	/* only works if obj is nil or inherits ... */
			/* ... /kernel/lib/user */

In the example above, the variable 'user1' can only hold an object which
inherits /kernel/lib/user.  To attempt to use a different type of object
results in a runtime error.  There is no compile-time checking for
object types.

This change will affect a great deal of code, so consider 1.2.89 to be
the last stable version for a while.

Regards,
Dworkin
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