feat(starlib): util.Marshal extended compatibility and tests

[mcuadros]

I was working on a encoding/yaml module and I found that util.Marshal lacks some types support. I added the coverage of this types plus a test for it.

Also I believe that can be useful introduce support for an interface like:

type StarkarkMarshaller interface {
   AsStarlark() starlark.Value
}

To allow conversion from any type and not just the standard ones.

[b5]

Looks great! Thanks for the additional tests & case types!

[b5]

Really into the idea of an AsStarlark interface @mcuadros. To me the question is where to put such an interface. It may warrant turning encoding into a go package that declares this interface. I'd love to think on it more, but agree AsStarlark is a great idea

[mcuadros]

Yep, could be worth do a library with Marshal, I have a similar code in my project as for sure many other people will ending doing the same kind of code.

[gebv]

Is StarkarkMarshaller that supported already? Is this issue stale?
Any ideas for Unmarshaller? Global registry for custom types (factroy)?

[b5]

hi @gebv! This is a merged pull request, the functionality described above is now part of the codebase, if you're in need of an Unmarshaler interface, we'd be delighted if you could file an issue describing your use case!