Segfault with "private" parameters inside "always" block

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

zsh/param/private assumes the heap created in startparamscope() is the
only active heap until endparamscope(), but "always" blocks allocate
and free another layer of heap, which invalidates the memory "private"
uses for the parameter GSU structures.

I haven't figured out what to do about this, yet.  The GSUs can't just
be static, because different special params use different callbacks
for get/set/unset.