Support persistent UAST hashes about sdk

[creachadair]

To avoid the issue with persistent data, I think there are two possible solutions:

1. Keep it opaque, but fix an algorithm and promise not to change it, or
2. Include a marker in the data that can be used to version it.

To implement (2), for example, we could make the first byte of the hash an algorithm tag (either by clobbering the first byte of the hash value, or just extending the array by 1 byte).

I don't really see much point in changing the algorithm, since we want it as a fingerprint rather than for security purposes, but either approach could work.