Use error handling for relocatable addition.

[alonh5]

This change is 

[alonh5]

From cairo-rs:

/// Warning: may panic if self.offset + rhs exceeds usize::MAX
impl AddAssign<usize> for Relocatable {
    fn add_assign(&mut self, rhs: usize) {
        self.offset += rhs
    }
}

Reviewable status: 0 of 3 files reviewed, all discussions resolved (waiting on @elintul and @spapinistarkware)

[elintul]

Thanks!

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @spapinistarkware)