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>

internal/plugins/statistics.go

@@ -6,7 +6,7 @@ import (
 	"time"
 
 	"git.maximhutz.com/max/lambda/internal/statistics"
-	"git.maximhutz.com/max/lambda/pkg/reducer"
+	"git.maximhutz.com/max/lambda/pkg/interpreter"
 )
 
 // An observer, to track reduction performance.
@@ -16,11 +16,11 @@ type Statistics struct {
 }
 
 // Create a new reduction performance Statistics.
-func NewStatistics(r reducer.Reducer) *Statistics {
+func NewStatistics(r interpreter.Interpreter) *Statistics {
 	plugin := &Statistics{}
-	r.On(reducer.StartEvent, plugin.Start)
-	r.On(reducer.StepEvent, plugin.Step)
-	r.On(reducer.StopEvent, plugin.Stop)
+	r.On(interpreter.StartEvent, plugin.Start)
+	r.On(interpreter.StepEvent, plugin.Step)
+	r.On(interpreter.StopEvent, plugin.Stop)
 
 	return plugin
 }