* Added `#:hold' to `_fun' for callbacks (will be changed to `#:keep'
soon, but better to have a checkpoint)
* _cprocedure also has a `hold' argument, and is now using keyword
arguments (it was getting crowded in the optional arguments
department)
* Documented everything that changed, with a longish descrption for
options for holding callback values.
* More tests, including tests for callbacks that would crash if the
callback values are not held.
svn: r11931
