Check-in [e3243a6107]

subclasses can be instantiated.

## New Definitions

The `classmethod` class definition creates a method that can be used when
invoked against its defining class or any of its subclasses.

The `classvariable` class definition creates a variable that exists in a
class's namespace and which is linked into any instance of that class
(including instances of subclasses); setting the variable with that name from
any of those instances or from the class will result in changes that can be
observed from all instances of that class.

The `initialise` class definition evaluates a script in a context where it can
access the class's namespace, allowing for easier initialisation of a class
than simply overriding its constructor.

## New Helper Commands

The `callback`/`mymethod` command, available within methods, takes the name of
a method and zero-or-more arguments and returns a script fragment that will
allow that method easy to invoke from a callback (e.g., a variable `trace`,
`chan event` callback, or Tk event binding). The command will be available
with both names.

The `classvariable` command, available within methods, binds a local variable
to a variable within the namespace of the class that defined the method.

The `link` command, available within methods, creates a binding for methods
so that calling the command with the given name is equivalent to calling
`my $name` instead. It can link multiple methods with one call, one per
argument, and those created commands can be renamed without losing the link;
if an argument is a two-element list, the first element is the name of the
method and the second is the name of the command (which will be resolved