starlark: Add Module type

[timothyg-stripe]

This PR adds a new Module type and surrounding API:

type Module struct{ … }
func (*Module) Program()     *Program
func (*Module) Globals()     StringDict
func (*Module) Predeclared() StringDict

func (*Program) InitModule(thread *Thread, predeclared StringDict) (*Module, error)

func (*Function) Module() *Module

The existing (*Program).Init function is reimplemented in terms of InitModule. Similarly, fn.Globals() is now the same as fn.Module().Globals().

Specifically, this adds a way to get the predeclared dictionary for a given Function, which makes it easier to write a debugger.

Fixes #594

[adonovan]

I wonder if it would make more sense to expose the Module type in the API (keeping its fields private), and to add the Predeclared method onto Module? That way it is clear that fn.Module().Predeclared() accesses a value that is shared among all functions of the same module (even if there's today nothing else you can do with the module--though in future there may be).

Also, unlike Globals, Predeclared should just return a reference to the StringDict, not a copy, since the user provided us with it in the first place.

[timothyg-stripe]

Should Globals live on this Module type too then?

[adonovan]

Yes, that seems appropriate, though of course we'll have to keep the existing method on Function as a wrapper.

[timothyg-stripe]

Done, PTAL.

[adonovan]

Test looks good; thanks.

[timothyg-stripe]

fixed all the comments

[adonovan]

LGTM, thanks again.