Rollup merge of #42082 - michaelwoerister:wider_def_path_hashes, r=eddyb

incr. comp.: Use more bits for DefPath hashes

Use 128 instead of 64 bits for DefPath hashes, like we do for everything else. Collision probability is unnecessarily high with 64 bits.

Also change the representation of `ich::Fingerprint` from `Fingerprint([u8; 16])` to `Fingerprint(u64, u64)` which is better for hashers like `FxHasher`.