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>

pkg/engine/normalorder/reduce_once.go

@@ -10,25 +10,25 @@ import "git.maximhutz.com/max/lambda/pkg/lambda"
 func ReduceOnce(e lambda.Expression) (lambda.Expression, bool) {
 	switch e := e.(type) {
 	case lambda.Abstraction:
-		body, reduced := ReduceOnce(e.Body())
+		body, reduced := ReduceOnce(e.Body)
 		if reduced {
-			return lambda.NewAbstraction(e.Parameter(), body), true
+			return lambda.Abstraction{Parameter: e.Parameter, Body: body}, true
 		}
 		return e, false
 
 	case lambda.Application:
-		if fn, fnOk := e.Abstraction().(lambda.Abstraction); fnOk {
-			return fn.Body().Substitute(fn.Parameter(), e.Argument()), true
+		if fn, fnOk := e.Abstraction.(lambda.Abstraction); fnOk {
+			return lambda.Substitute(fn.Body, fn.Parameter, e.Argument), true
 		}
 
-		abs, reduced := ReduceOnce(e.Abstraction())
+		abs, reduced := ReduceOnce(e.Abstraction)
 		if reduced {
-			return lambda.NewApplication(abs, e.Argument()), true
+			return lambda.Application{Abstraction: abs, Argument: e.Argument}, true
 		}
 
-		arg, reduced := ReduceOnce(e.Argument())
+		arg, reduced := ReduceOnce(e.Argument)
 		if reduced {
-			return lambda.NewApplication(e.Abstraction(), arg), true
+			return lambda.Application{Abstraction: e.Abstraction, Argument: arg}, true
 		}
 
 		return e, false