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/explanation.go

@@ -5,19 +5,19 @@ package plugins
 import (
 	"fmt"
 
-	"git.maximhutz.com/max/lambda/pkg/reducer"
+	"git.maximhutz.com/max/lambda/pkg/interpreter"
 )
 
 // Track the reductions made by a reduction process.
 type Explanation struct {
-	reducer reducer.Reducer
+	reducer interpreter.Interpreter
 }
 
 // Attaches a new explanation tracker to a reducer.
-func NewExplanation(r reducer.Reducer) *Explanation {
+func NewExplanation(r interpreter.Interpreter) *Explanation {
 	plugin := &Explanation{reducer: r}
-	r.On(reducer.StartEvent, plugin.Start)
-	r.On(reducer.StepEvent, plugin.Step)
+	r.On(interpreter.StartEvent, plugin.Start)
+	r.On(interpreter.StepEvent, plugin.Step)
 
 	return plugin
 }