Include AssignTree module

[danmatichuk]

The ASL translator defines an AssignTree datatype, which is a generalization of an Assignment to trees, with a corresponding CtxTree type parameter for a tree of contexts.

This turns out to be useful for describing recursive types (e.g. What4 expressions) where you want to reason about operations that manipulate the structure of the type (e.g. flattening a nested What4 struct).

https://github.com/GaloisInc/asl-translator/blob/feature/ctxfuns/lib/Data/Parameterized/AssignTree.hs