The abstract semantic graph (ASG) for a Leo program.
This module contains the [Asg] type, an abstract data type that represents a Leo program
as a series of graph nodes. The [Asg] type is at a greater level of abstraction than an [Ast].
A new [Asg] type can be created from an [Ast].
Converting to an [Asg] provides greater type safety by canonicalizing and checking program types.