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>

internal/registry/codec.go

@@ -7,18 +7,24 @@ import (
 	"git.maximhutz.com/max/lambda/pkg/codec"
 )
 
+// A Codec is a type-erased codec that serializes and deserializes expressions
+// as Expr values, regardless of the underlying representation type.
 type Codec interface {
 	codec.Codec[Expr]
 
+	// InType returns the name of the representation this codec handles.
 	InType() string
 }
 
-type convertedCodec[T any] struct {
+// A registeredCodec adapts a typed codec.Codec[T] into the type-erased Codec
+// interface. It wraps decoded values into Expr on decode, and extracts the
+// underlying T from an Expr on encode.
+type registeredCodec[T any] struct {
 	codec  codec.Codec[T]
 	inType string
 }
 
-func (c convertedCodec[T]) Decode(s string) (Expr, error) {
+func (c registeredCodec[T]) Decode(s string) (Expr, error) {
 	t, err := c.codec.Decode(s)
 	if err != nil {
 		return nil, err
@@ -27,7 +33,7 @@ func (c convertedCodec[T]) Decode(s string) (Expr, error) {
 	return NewExpr(c.inType, t), nil
 }
 
-func (c convertedCodec[T]) Encode(r Expr) (string, error) {
+func (c registeredCodec[T]) Encode(r Expr) (string, error) {
 	t, ok := r.Data().(T)
 	if !ok {
 		dataType := reflect.TypeOf(r.Data())
@@ -38,13 +44,15 @@ func (c convertedCodec[T]) Encode(r Expr) (string, error) {
 	return c.codec.Encode(t)
 }
 
-func (c convertedCodec[T]) InType() string { return c.inType }
+func (c registeredCodec[T]) InType() string { return c.inType }
 
+// RegisterCodec registers a typed codec under the given representation name.
+// Returns an error if a codec for that representation is already registered.
 func RegisterCodec[T any](registry *Registry, m codec.Codec[T], inType string) error {
 	if _, ok := registry.codecs[inType]; ok {
 		return fmt.Errorf("Codec for '%s' already registered", inType)
 	}
 
-	registry.codecs[inType] = convertedCodec[T]{m, inType}
+	registry.codecs[inType] = registeredCodec[T]{m, inType}
 	return nil
 }