Usage of DefaultHasher

[ernieIzde8ski]

The OG genanki has a(n admittedly strange & somewhat broken) hash algorithm implemented from scratch, which, as far as I can tell, solves one issue: Python hashes are salted, and so are not consistent between runs. Similarly, genanki-rs uses DefaultHasher, which is not guaranteed to have a consistent implementation between compiler versions. This makes the implementation of guid_for in genank-rs unstable for repeated builds.