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/scan.go

@@ -6,7 +6,6 @@ import (
 	"unicode"
 
 	"git.maximhutz.com/max/lambda/pkg/iterator"
-	"git.maximhutz.com/max/lambda/pkg/trace"
 )
 
 // isVariables determines whether a rune can be a valid variable.
@@ -51,38 +50,38 @@ func scanToken(i *iterator.Iterator[rune]) (*Token, error) {
 
 	letter, err := i.Next()
 	if err != nil {
-		return nil, trace.Wrap(err, "cannot produce next token")
+		return nil, fmt.Errorf("cannot produce next token: %w", err)
 	}
 
 	switch {
 	case letter == '(':
-		return NewTokenOpenParen(index), nil
+		return NewToken(TokenOpenParen, index), nil
 	case letter == ')':
-		return NewTokenCloseParen(index), nil
+		return NewToken(TokenCloseParen, index), nil
 	case letter == '.':
-		return NewTokenDot(index), nil
+		return NewToken(TokenDot, index), nil
 	case letter == '\\':
-		return NewTokenSlash(index), nil
+		return NewToken(TokenSlash, index), nil
 	case letter == '\n':
-		return NewTokenSoftBreak(index), nil
+		return NewToken(TokenSoftBreak, index), nil
 	case letter == '{':
-		return NewTokenOpenBrace(index), nil
+		return NewToken(TokenOpenBrace, index), nil
 	case letter == '}':
-		return NewTokenCloseBrace(index), nil
+		return NewToken(TokenCloseBrace, index), nil
 	case letter == ':':
 		if _, err := scanCharacter(i, '='); err != nil {
 			return nil, err
 		} else {
-			return NewTokenAssign(index), nil
+			return NewToken(TokenAssign, index), nil
 		}
 	case letter == ';':
-		return NewTokenHardBreak(index), nil
+		return NewToken(TokenHardBreak, index), nil
 	case letter == '#':
 		// Skip everything until the next newline or EOF.
 		for !i.Done() {
 			r, err := i.Next()
 			if err != nil {
-				return nil, trace.Wrap(err, "error while parsing comment")
+				return nil, fmt.Errorf("error while parsing comment: %w", err)
 			}
 
 			if r == '\n' {