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/expr.go

@@ -1,24 +1,26 @@
 package registry
 
-// A Expr is a lambda calculus expression. It can have any type of
-// Expresentation, so long as that class is known to the registry it is handled
+// An Expr is a type-erased lambda calculus expression. It can have any type of
+// representation, so long as that type is known to the registry it is handled
 // by.
 type Expr interface {
-	// Repr returns the name of the underlying Expresentation. It is assumed if
-	// two expressions have the same Repr(), they have the same Expresentation.
+	// Repr returns the name of the underlying representation. Two expressions
+	// with the same Repr() are assumed to have the same representation type.
 	Repr() string
 
-	// The base expression data.
+	// Data returns the underlying expression data.
 	Data() any
 }
 
+// A baseExpr is the default implementation of Expr.
 type baseExpr struct {
 	id   string
 	data any
 }
 
-func (r baseExpr) Repr() string { return r.id }
+func (e baseExpr) Repr() string { return e.id }
 
-func (r baseExpr) Data() any { return r.data }
+func (e baseExpr) Data() any { return e.data }
 
+// NewExpr creates an Expr with the given representation name and data.
 func NewExpr(id string, data any) Expr { return baseExpr{id, data} }