validate_args: a quick and dirty required-and-optional arg checker

[rjbs]

This isn't perfect, but it will be a quick way for lots of things to start off by saying "if there are unknown or missing arguments, just give up." Right now, we see this pattern repeated as too many identical-looking lines of code throughout different method handlers.

[mmcclimon]

One tiny comment, otherwise looks good.

[wolfsage]

I like it. Seems like something Ix should be able to use and auto-generate validators based on metadata... maybe...

[wolfsage]

I think one of the more dangerous elements is getting references when we expect strings. A type-enforcing validator would go a long way

[rjbs]

My plan is to have a future form where you can say:

{ required => { foo => $validator } }

...but I didn't want to get bogged down in that yet.

[rjbs]

Okay, I went and got bogged down in that. It's nice.

[wolfsage]

I like it.

[rjbs]

Applied.