Fix checksum portability

[aleksa2808]

This PR fixes checksum portability across different architectures by:

1. Replacing the usage of usize with u64 in places where these numbers were hashed.
2. Switching the hasher implementation from the Bevy-provided, non-portable AHash hasher to the portable seahash hasher.

Fixes #98.

[johanhelsing]

Thanks! If someone wants to add a test for this, we could perhaps look into using the insta for snapshot testing of bevy_ggrs snapshot checksum values, and run test on two different architectures.

[johanhelsing]

Or just hardcode the checksum value in the test I guess, they're not that long...