Right now to parse code the embedder has to pass a CompileOptions + SourceText to one of the APIs. The frontend then allocates the ScriptSource and ScriptSourceObject at some point. All this gets more complicated with XDR and off-thread parsing/decoding.

We should consider adding an API (JS::CreateScriptSource) to create the ScriptSource{Object} explicitly, upfront (it would take the filename, lineno and other things currently stored in CompileOptions). This would shrink CompileOptions and make it much easier to reason about ScriptSource allocation.