Callbacks via composability

[mitschabaude]

• Closes Dex: Internals continued #402
• Enables to pass AccountUpdates to methods (by making the class implement Provable<AccountUpdate>)
• Add an experimental authorize method, which can (should) be used on AccountUpdates that are passed to methods:

@method authorizingMethod(accountUpdate: AccountUpdate) {
  this.experimental.authorize(accountUpdate);
}

In this case, authorize simply takes the AccountUpdate and makes it a child of the zkApp ("adopts it").

There's also an advanced second parameter to witness a certain layout of child updates:

this.experimental.authorize(accountUpdate, [0, 1, [null]]);

The weird layout syntax is documented here:
https://github.com/o1-labs/snarkyjs/blob/2bf421ba36780c4dc644552e02a6d2dcfe51036c/src/lib/account_update.ts#L1211-L1230

authorize can be used to achieve what the callback was all about: instead of taking a "zkApp callback" and extracting the account update in the authorizing method, we let it be up to the method's caller to get the account update, and pass it in directly.
The caller could, for example, do this

• call a third zkApp
• take the account update off of thirdZkapp.self
• pass that account update to a method like authorizingMethod above, which uses authorize (and then does checks on the witnessed account update tree, like summing up the balances)

If this is done, the functionality is equivalent to a callback, with the difference that the caller can also see the result of the callback! (which, it turns out, is what we often want)

Let's call this flow "manual callback".

The "manual callback" flow is safer than the old callback, because our zkApps composability machinery ensures that we properly prove in the caller's circuit that thirdZkapp was really called, with certain arguments and outputs.

This PR also:

• Refactors the DEX to use "manual callback" instead of callbacks
• Refactor callbacks to use the "manual callback" flow under the hood, to be secure
• Adds helpers for pretty-printing a transaction or account update, with .toPretty()
• Unrelatedly, makes some tweaks to the ./run script
• Unrelatedly, tweaks how Circuit.witness works

[MartinMinkov]

Pre approving! Looks great, awesome work 🚀