[Roadmap] Constraints Library

[johnyob]

Issues related to Dromedary's Constraints library.

• Split Constraints library into Constraints and Unifier #44
• Reset counters for each invocation of Constraints.solve #47
• Add error handling into the Constraints library
• Add transactions ("Snapshots") to Unifier
• Add monadic primitives to Constraints library

Potential (less concrete) issues include:

• Abstracting Constraints further to avoid reliance on generalization, by abstracting the solving of first-order let abstractions.