Generates a v5 UUID as witness ID based on stream ID and seq.

These need to be constructors, rather than a global var that's reused, so that there is not a race condition when marshaling to protobufs that share them.

code adopted from https://github.com/ShiraazMoollatjie/goluhn.

This limit is used for non-YAML and non-JSON types that we can have some hope of parsing.

The fallback to trying compression algorithms is more exprensive because there doesn't seem to be a good way of interrogating the algorithms about whether the stream is OK.

For types where we just return a string (or maybe an int) then it doesn't make sense to pull in a lot of data, just to hash it anyway.

A partial witness is a witness that only includes the argument or response.