add pic_throw function
error function is no longer used
add notation about pic_try usage
drop support for raise-continuable for a moment
update readme
don't use pic->rescue stack in with-exception-handler
pic_raise is equivalent to pic_throw. remove it.
inline pic_error definition
cleanup
pic->rescue is no longer used
remove error handling stuff from the VM
pic_gc_protect now returns a value
cleanup
refactor macroexpand
use UNREACHABLE macro
throw error when macro definition evaluates to non-procedure object
Merge branch 'refactor-error'