Implement `dolt prove`

[timsehn]

From Discord with a potential user:

It would be great (more from a blockchainish nonrepudiable evidence of state perspective than for gittish versioning applications) if there were some way to climb the tree from a piece of relevant state to the root hash.

Then me:

I guess it would be dolt prove <table> <primary key(s)>

Then user:

with a third optional argument so you could specify a past state root.

Running the command would spit out the exact hash of the database page (chunk in Dolt lingo) and then the hash path to root.

You could strings the chunk file in .dolt if you wanted to actually see the value on. disk. Or we could implement a way to read these files directly.