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/converter.go

@@ -1,13 +1,18 @@
 package registry
 
+// A Converter is a directed graph of conversions between representations. Each
+// node is a representation name, and each edge is a Conversion.
 type Converter struct {
 	data map[string][]Conversion
 }
 
+// NewConverter creates an empty Converter with no registered conversions.
 func NewConverter() *Converter {
 	return &Converter{data: map[string][]Conversion{}}
 }
 
+// Add registers a conversion, adding an edge from its source representation
+// to its target representation.
 func (g *Converter) Add(c Conversion) {
 	conversionsFromIn, ok := g.data[c.InType()]
 	if !ok {
@@ -18,6 +23,8 @@ func (g *Converter) Add(c Conversion) {
 	g.data[c.InType()] = conversionsFromIn
 }
 
+// ConversionsFrom returns all conversions that have the given representation
+// as their source type.
 func (g *Converter) ConversionsFrom(t string) []Conversion {
 	return g.data[t]
 }