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/saccharine/codec.go

@@ -1,14 +1,15 @@
-// Package "saccharine" provides a simple language built on top of λ-calculus,
-// to facilitate productive coding using it.
 package saccharine
 
 import (
 	"git.maximhutz.com/max/lambda/pkg/codec"
 )
 
-type Marshaler struct{}
+// A Codec is a [codec.Codec] that serializes Saccharine expressions.
+type Codec struct{}
 
-func (m Marshaler) Decode(s string) (Expression, error) {
+// Decode parses a string as Saccharine source code. Returns an error
+// if it cannot.
+func (c Codec) Decode(s string) (Expression, error) {
 	tokens, err := scan(s)
 	if err != nil {
 		return nil, err
@@ -17,8 +18,10 @@ func (m Marshaler) Decode(s string) (Expression, error) {
 	return parse(tokens)
 }
 
-func (m Marshaler) Encode(e Expression) (string, error) {
+// Encode turns a Saccharine expression into a string. Returns an error if it
+// cannot.
+func (c Codec) Encode(e Expression) (string, error) {
 	return stringifyExpression(e), nil
 }
 
-var _ codec.Codec[Expression] = (*Marshaler)(nil)
+var _ codec.Codec[Expression] = (*Codec)(nil)