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/performance.go

@@ -7,7 +7,7 @@ import (
 	"path/filepath"
 	"runtime/pprof"
 
-	"git.maximhutz.com/max/lambda/pkg/reducer"
+	"git.maximhutz.com/max/lambda/pkg/interpreter"
 )
 
 // Observes a reduction process, and publishes a CPU performance profile on
@@ -19,10 +19,10 @@ type Performance struct {
 }
 
 // Create a performance tracker that outputs a profile to "file".
-func NewPerformance(file string, process reducer.Reducer) *Performance {
+func NewPerformance(file string, process interpreter.Interpreter) *Performance {
 	plugin := &Performance{File: file}
-	process.On(reducer.StartEvent, plugin.Start)
-	process.On(reducer.StopEvent, plugin.Stop)
+	process.On(interpreter.StartEvent, plugin.Start)
+	process.On(interpreter.StopEvent, plugin.Stop)
 
 	return plugin
 }