[libcroco-list] cr-declaration enhancements

[rwlbuis xs4all nl]

Hi,

I did some further enhancements for cr-declaration :

- I copied and pasted the g_return_if_fail code for the three
new functions from my last patch, while I actually needed
g_return_val_if_fail, this is fixed in this new patch.
- To serialize a decl list (as opposed to just 1 decl) I added
a new function. I am not 100% sure about mem usage, so please check...
Comments?
Cheers,

Rob.