Working mutex solution

It's not nearly as clean as I'd like it to be, but I've finally
conceived code that should act the way the feature was intended to.
Though a previous revision "worked," it wasn't managed at an instance
level.  This one is.  Synchronized objects that have invoked the
#mutex() method will declare the instance variable @__locks__, which is
a Symbol=>Monitor Hash that contains the mutex locks for each registered
attribute.

For example, calling mutex( :sample ) will create the @sample instance
variable unless it already exists and add a wrapped writer method
(:sample=) that sets the variable inside the lock's #synchronize()
block.

Other than the new pattern for declaration within the #initialize()
method of an implementing class (or elsewhere in a module), access and
modification of variables remains otherwise the same.

(Note:  I, @florf, may be taking an extended break from development for
the time being.)