Make `Until` consistent with PJ paper, change american option builder. Fixes 69

[matteolimberto-da]

Fixes https://github.com/digital-asset/contingent-claims/issues/69

This is the PR where the dynamics of Until as well as the american option builder are changed. The problem and proposed solution are described in the related issue.

What I am not yet happy with is the fact that acquire' is now a very complex functions, as it

• evaluates When andAnytimepredicates
• fixes acquisition time of non-deterministic predicates
• evaluates Cond predicates
• consumes Cond nodes once they are reached
• expires Until nodes when the predicate is True
• expires worthless sub-trees (When and Anytime node where the predicate will never be met)

This can probably be split in different functions to make it a bit simpler. lifecycle would then be a composition of these simpler functions.

[matteolimberto-da]

The latest commits decompose the acquire' function in three components:

• non-effectful acquire' : its job is to acquire the sub-trees and calculate their acquisition time
• expire' : expires sub-trees that will never be acquired
• fixAcquisitionTime' : fixes the acquisition time for When nodes with a non-deterministic predicate

I think this makes the code easier to read.

I also got rid of expire as it is no longer needed.

An advantage of the decomposition is that now we can make exercise use only the non-effectful acquire' and thus address https://github.com/digital-asset/contingent-claims/issues/73

[matteolimberto-da]

As discussed in person this seems fine to me. The only thing I would think about is having clear combinators for distinguishing time =< >= and decimal. at and upTo sound fine though.

Thank you. Will open an issue to discuss the names within the team.