Renaming of val_[to|of]int into val_[to|of]_Z.

This was done using the following sed commands (with ZSH ** pattern):
> sed -i 's/val_of_int/val_of_Z/g' **/*.v
> sed -i 's/val_to_int/val_to_Z/g' **/*.v
2 jobs for ci/refactoring in 19 minutes and 38 seconds (queued for 3 seconds)