Standardize using ExtendedTime rather than base time in all API functions

[elidupree]

I've been steadily exposing ExtendedTime more and more, and at this point, there's no reason not to go the rest of the way.

This applies to snapshot_before(), valid_since(), updated_until_before(), and forget_before().

This is mostly just an elegance thing, but it may be useful to allow snapshots of unusual ExtendedTimes for debug-examining stuff.

insert_fiat_event() could still use base time + id, because it's similar to the interface for creating predictions, and it supports automatically protecting the user from colliding fiat event ids with prediction ids.

The main downside of this change is that it would obligate the user to call beginning_of() themselves, but that seems tolerable.

[elidupree]

Correction: valid_since() defines the times when it's legal to create fiat events, which can only be in increments of base time. However, it still makes sense for the snapshot function, updated_until_before(), and forget_before() to support ExtendedTime.

Also, the snapshot function wouldn't be called snapshot_before() anymore, but rather just snapshot().

[elidupree]

My latest version of the design has eliminated the iteration concept and the ExtendedTime type.

The question remains of whether mid-instant snapshots make sense. They are certainly a confusing concept, so I'm inclined to say that even if they exist, they are a separate function from snapshot_before().