Allow disjuncts in proposals

[rowgraus]

What is the Problem Being Solved?

Needed in Dominant Assurance Contracts: As a user, I want to specify that either the public good is produced, OR I get money back plus the extra amount I get for participating (note that this is different than what I gave).

Description of the Design

Security Considerations

Test Plan

[erights]

Also would be a natural extension of the pattern notion explored in the #1905 spike, if that works out. More involved that #1913 but also more widely useful.

[katelynsills]

This should be solved by using an or pattern in your want in the proposal. We should add this to the use cases for that task. #2230

[erights]

This should be solved by using an or pattern in your want in the proposal. We should add this to the use cases for that task. #2230

Having amounts with different brands on each branch is the use case I have in mind for why the pattern language relies on self-describing amount arithmetic.