The current error handling scheme is a weird mix of Rust error handling and Scheme error handling. For example, the walk method checks if our substitution map is an Err and returns Sym("nil") if it is. It seems like it would be preferential to simply use Results everywhere instead.

Because a State is fully self-contained, there should never be a reason to copy data owned by a State without copying the State itself (at least until reification). The substitution map effectively serves as an arena allocator, so operations that currently require copying could instead operate on indices. LVars are exactly these indices, but they have their own semantics that may complicate using them as a generic "reference into State" type.