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/saccharine.go

@@ -0,0 +1,60 @@
+// Package saccharine defines the AST for the Saccharine language, a sugared
+// lambda calculus with let bindings and multi-statement clauses.
+package saccharine
+
+// An Expression is a node in the Saccharine abstract syntax tree.
+// It is a sealed interface; only types in this package may implement it.
+type Expression interface {
+	expression()
+}
+
+// An Abstraction is a lambda expression with zero or more parameters.
+// A zero-parameter abstraction is treated as a thunk.
+type Abstraction struct {
+	Parameters []string
+	Body       Expression
+}
+
+// An Application applies an expression to zero or more arguments.
+type Application struct {
+	Abstraction Expression
+	Arguments   []Expression
+}
+
+// An Atom is a named variable.
+type Atom struct {
+	Name string
+}
+
+// A Clause is a sequence of statements followed by a return expression.
+type Clause struct {
+	Statements []Statement
+	Returns    Expression
+}
+
+func (Abstraction) expression() {}
+func (Application) expression() {}
+func (Atom) expression()        {}
+func (Clause) expression()      {}
+
+// A Statement is a declaration within a Clause.
+// It is a sealed interface; only types in this package may implement it.
+type Statement interface {
+	statement()
+}
+
+// A LetStatement binds a name (with optional parameters) to an expression.
+type LetStatement struct {
+	Name       string
+	Parameters []string
+	Body       Expression
+}
+
+// A DeclareStatement evaluates an expression for its side effects within a
+// clause.
+type DeclareStatement struct {
+	Value Expression
+}
+
+func (LetStatement) statement()     {}
+func (DeclareStatement) statement() {}