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/codec/codec.go

@@ -1,8 +1,20 @@
+// Package codec defines processes to convert between different representations
+// of lambda calculus, and serialize the different representations.
 package codec
 
+// A Conversion is a function that turns one representation into another.
+// Returns an error if the input expression cannot be converted.
 type Conversion[T, U any] = func(T) (U, error)
 
+// A Codec is an object that can serialize/deserialize one type of
+// representation. It is assumed that for any x ∋ T, Decode(Encode(x)) = x.
 type Codec[T any] interface {
+	// Encode takes an expression, and returns its serialized format, as a
+	// string. Returns an error if the expression cannot be serialized.
 	Encode(T) (string, error)
+
+	// Decode takes the serialized format of an expression, and returns its true
+	// value. Returns an error if the string doesn't correctly represent any
+	// valid expression.
 	Decode(string) (T, error)
 }