refactor: make lambda expression types immutable

- 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
- Remove pointer equality optimizations (not applicable with immutable values)
- Return empty set instead of nil from GetFreeVariables default case

internal/plugins/debug.go

@@ -3,17 +3,17 @@ package plugins
 import (
 	"log/slog"
 
-	"git.maximhutz.com/max/lambda/pkg/reducer"
+	"git.maximhutz.com/max/lambda/pkg/interpreter"
 )
 
 type Logs struct {
 	logger  *slog.Logger
-	reducer reducer.Reducer
+	reducer interpreter.Interpreter
 }
 
-func NewLogs(logger *slog.Logger, r reducer.Reducer) *Logs {
+func NewLogs(logger *slog.Logger, r interpreter.Interpreter) *Logs {
 	plugin := &Logs{logger, r}
-	r.On(reducer.StepEvent, plugin.Step)
+	r.On(interpreter.StepEvent, plugin.Step)
 
 	return plugin
 }