add lazy-import

[AlexKnauth]

This allows me to lazy-import certain libraries to be able to compile glow on WSL.

[AlexKnauth]

@fare Although Glow-Lang/glow#419 is no longer necessary, I still think lazy-import could be a good addition to gerbil-utils on its own.

[fare]

Maybe it could make it directly upstream in Gerbil? What does vyzo think? There would have to be more documentation, though.

[fare]

Looking at the code, it seems that it's only for functions. Is that the case?
That requires documentation, to say the least. Maybe a runtime check, if meaningful—also a check that the binding is present and of the right kind, etc.

In another way, the syntax looks like it is heavy in parentheses. Is there a good reason for that?

[AlexKnauth]

Yes, it's only for functions, inspired by how Racket's lazy-require is only for functions. That's documented here for lazy-require:

Defines each fun-id as a function that, when called, dynamically requires the export

[AlexKnauth]

The inspiration from Racket's lazy-require is also the reason for the parentheses being as "heavy" as they are.

[fare]

OK, but since we're not compatible anyway (due to it being called "lazy-import" rather than "lazy-require", and hopefully having the same syntax as import for naming modules as either symbols or strings (or isn't the latter the case?), can we skip the extra layer of parentheses around the names of the imported functions? Or going the other way, we could triple or quadruple the number of parentheses just to be sure.