Redesign local-variable binding mechanism so that default global value

Redesign local-variable binding mechanism so that default global value
is always stored in a special slot in the variable object.  Variable
assignment daemons are now called only when the value changes in some
perceptible way, not when the local bindings are swapped in or out of
the value cache.  Also, variable assignment daemons now take an
additional argument, either a buffer or #F, indicating whether the
change is local or global.