refactor: make lambda expression types immutable (#38)

## Summary

- Change Abstraction, Application, and Variable to use private fields with getter methods.
- Return value types instead of pointers from constructors.
- Update all type switches to match value types instead of pointer types.

## Test plan

- [x] All existing tests pass (`make test`).

Reviewed-on: #38
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cmd/lambda/lambda.go

@@ -34,34 +34,34 @@ func main() {
 	logger.Info("compiled λ expression", "tree", compiled.String())
 
 	// Create reducer with the compiled expression.
-	reducer := lambda.NewNormalOrderReducer(&compiled)
+	interpreter := lambda.NewInterpreter(compiled)
 
 	// If the user selected to track CPU performance, attach a profiler.
 	if options.Profile != "" {
-		plugins.NewPerformance(options.Profile, reducer)
+		plugins.NewPerformance(options.Profile, interpreter)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
 	// observer.
 	if options.Explanation {
-		plugins.NewExplanation(reducer)
+		plugins.NewExplanation(interpreter)
 	}
 
 	// If the user opted to track statistics, attach a tracker.
 	if options.Statistics {
-		plugins.NewStatistics(reducer)
+		plugins.NewStatistics(interpreter)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		plugins.NewLogs(logger, reducer)
+		plugins.NewLogs(logger, interpreter)
 	}
 
 	// Run reduction.
-	reducer.Reduce()
+	interpreter.Run()
 
 	// Return the final reduced result.
-	result := reducer.Expression().String()
+	result := interpreter.Expression().String()
 	err = options.Destination.Write(result)
 	cli.HandleError(err)
 }

cmd/lambda/lambda_test.go

@@ -30,8 +30,8 @@ func runSample(samplePath string) (string, error) {
 	compiled := convert.SaccharineToLambda(ast)
 
 	// Create and run the reducer.
-	reducer := lambda.NewNormalOrderReducer(&compiled)
-	reducer.Reduce()
+	reducer := lambda.NewInterpreter(compiled)
+	reducer.Run()
 
 	return reducer.Expression().String() + "\n", nil
 }