Check-in [96cdaa7bf6]

Again, note that creation IDs are _always_ system-allocated and are _never_
guaranteed to be unique between interpreters, either in multiple processes or
in the same thread or process; they are only ever locally unique.

# Example

    oo::class create LabelEqual {
        constructor {label} {
            set [my varname label] $label
        }
        private {
            variable label
            method getLabel {} {
                return $label
            }
        }
        method equals {other} {
            expr {$label eq [$other getLabel]}
        }
    }

    oo::class create Evaluated {
        superclass LabelEqual
        # Poorly chosen variable name! Happens too easily in real life
        variable label
        constructor {expression} {
            next $expression
            set label [expr $expression]
        }
        method value {} {
            return $label
        }
    }

    set expr1 [Evaluated new {1 + 2 + 3}]
    set expr2 [Evaluated new {3 + 2 + 1}]
    puts "one is two? [$expr1 equals $expr2]"
    # Prints: one is two? 0
    puts "one=[$expr1 value] two=[$expr2 value]"
    # Prints: one=6 two=6
    puts [info vars [info object namespace $expr1]::*]
    # Prints something like: {::oo::Obj13::11 : label} ::oo::Obj13::label
    catch {$expr2 getLabel} msg
    puts $msg
    # Prints: unknown method "getLabel": must be destroy, equals or value

# Implementation

See the [`tip-500` branch](https://core.tcl.tk/tcl/timeline?r=tip-500).

# Copyright