blake3 is a variable output algorithm, so is this blake3-256?

[rchincha]

https://github.com/opencontainers/go-digest/blob/22b78e47854adc56477927aba5f4c930b56af8c9/algorithm.go#L98

https://github.com/zeebo/blake3/blob/master/api.go#L17 ^ does appear to default to 256-bit output, but would still make this mnemonic explicit