docs: document remaining packages and simplify AST types (#45)

## Summary

- Added doc comments across the codebase: `pkg/lambda`, `pkg/saccharine`, `pkg/codec`, `pkg/engine`, `pkg/iterator`, `pkg/set`, `pkg/convert`, `internal/registry`, and `cmd/lambda`.
- Made lambda and saccharine expression structs use public fields instead of getters, matching `go/ast` conventions.
- Removed superfluous constructors for saccharine and lambda expression/statement types in favor of struct literals.
- Consolidated saccharine token constructors into a single `NewToken` function.
- Removed the unused `trace` package.

## Test plan

- [x] `go build ./...` passes.
- [x] `go test ./...` passes.
- [ ] Verify `go doc` output renders correctly for documented packages.

Reviewed-on: #45
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cmd/lambda/lambda.go

@@ -1,3 +1,4 @@
+// Package main defines the 'lambda' command-line interface (CLI).
 package main
 
 import (

cmd/lambda/lambda_engine.go

@@ -9,7 +9,7 @@ func LambdaEngine() *cobra.Command {
 		Use:     "engine",
 		Aliases: []string{"eng"},
 		Short:   "Information about available engines",
-		RunE: func(cmd *cobra.Command, args []string) error {
+		RunE: func(cmd *cobra.Command, _ []string) error {
 			return cmd.Help()
 		},
 	}

cmd/lambda/lambda_engine_list.go

@@ -11,7 +11,7 @@ func LambdaEngineList() *cobra.Command {
 		Use:     "list",
 		Aliases: []string{"ls"},
 		Short:   "List available engines",
-		RunE: func(cmd *cobra.Command, args []string) error {
+		RunE: func(*cobra.Command, []string) error {
 			r := GetRegistry()
 
 			for engine := range r.ListEngines() {

cmd/lambda/registry.go

@@ -19,8 +19,8 @@ func GetRegistry() *registry.Registry {
 	(registry.RegisterEngine(r, normalorder.NewProcess, "normalorder", "lambda"))
 
 	// Marshalers
-	(registry.RegisterCodec(r, lambda.Marshaler{}, "lambda"))
-	(registry.RegisterCodec(r, saccharine.Marshaler{}, "saccharine"))
+	(registry.RegisterCodec(r, lambda.Codec{}, "lambda"))
+	(registry.RegisterCodec(r, saccharine.Codec{}, "saccharine"))
 
 	return r
 }