Add assertf and use in boot.janet. Address #1516

[sogaiu]

This is an attempt to address #1516 by adding assertf (borrowed from @ianthehenry) to boot.janet.

The PR also includes attempts to apply assertf within boot.janet. I've used %v in some places but I'm not sure if that was appropriate.

Also, there was one bit of code that used errorf (may be as a short-cut?):

(assert bundle-name (errorf "unable to infer bundle name for %v, use :name argument" path))

this was replaced with:

(assertf bundle-name "unable to infer bundle name for %v, use :name argument" path)

I hope that was appropriate (^^;

For background, please see the Zulip discussion and the corresponding issue.

[pepe]

It looks fine, do you plan to add tests for it?

[sogaiu]

I totally didn't think of it (^^;

Thanks for the reminder.

Added some things. Not sure how appropriate they are. How do they look?

[pepe]

LGTM

[sogaiu]

Thanks!