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/process.go

@@ -4,28 +4,32 @@ import (
 	"git.maximhutz.com/max/lambda/pkg/engine"
 )
 
+// A Process is a type-erased reduction process that operates on Expr values.
 type Process interface {
 	engine.Process[Expr]
 
+	// InType returns the name of the representation this process operates on.
 	InType() string
 }
 
-type convertedProcess[T any] struct {
+// A registeredProcess adapts a typed engine.Process[T] into the type-erased
+// Process interface. It wraps the result of Get into an Expr.
+type registeredProcess[T any] struct {
 	process engine.Process[T]
 	inType  string
 }
 
-func (e convertedProcess[T]) InType() string { return e.inType }
+func (p registeredProcess[T]) InType() string { return p.inType }
 
-func (b convertedProcess[T]) Get() (Expr, error) {
-	s, err := b.process.Get()
+func (p registeredProcess[T]) Get() (Expr, error) {
+	s, err := p.process.Get()
 	if err != nil {
 		return nil, err
 	}
 
-	return NewExpr(b.inType, s), nil
+	return NewExpr(p.inType, s), nil
 }
 
-func (b convertedProcess[T]) Step(i int) bool {
-	return b.process.Step(i)
+func (p registeredProcess[T]) Step(i int) bool {
+	return p.process.Step(i)
 }