fn resolve_assign(&mut self, unique_symbol: UniqueSymbol) {
        let index = if let Some(i) = self.scope.local_index(unique_symbol) {
            self.lambda.emit(Opcode::Save);
            i
        } else if let Some(i) = self.scope.nonlocal_index(unique_symbol) {
            self.lambda.emit(Opcode::SaveCap);
            i
        } else {
            // unreachable?
            todo!()
        };

        self.lambda.emit_bytes(&mut split_number(index));
    }
Resolves the assignment of a variable
returns true if the variable was declared.
passerine/src/compiler/compile.rs