Switch to BLAKE3 for checksums

[programmerlexi]

BLAKE3 is a much more efficient algorithm for computing hashes on the host, when compared to BLAKE2b.

This additional speed is quite important when hashing several large files for the config.

[iczelia]

The biggest difficulty that I see right now is that it would be pretty annoying to implement backwards compatibility for this. I will try to take a stab at this sometime next week.

[Zesko]

My idea is to keep the hash symbol # unchanged for both blake2b and blake3.
The hash length alone determines which algorithm is used: 128 chars = blake2b, 64 chars = blake3.

This preserves backwards compatibility because snapper-sync tool checks for the # symbol, not the hash length.
Changing the # symbol would break compatibility.

[iczelia]

Implemented on trunk.