Replace ValueCow::as_bool with truthiness function

[lunabunn]

Not sure if you actually wanted this, but I felt that it could be useful for my application, so:
Implements a truthiness function for evaluating conditionals.

The following values are falsy:

• None
• false
• 0 integer and 0.0 float
• empty string, list, and map

Everything else is truthy.

Closes #9. Please let me know if there is anything you would like to see changed!

[rossmacarthur]

I think what I didn't put in the original issue #9 is that imagined the truthiness to be configurable. I think my original thoughts were that it makes template logic more robust to enforce booleans in if statements. Although I don't have any real data to back this up, if you think this is what most people will want then I'm happy to make this the default.

[lunabunn]

I think what I didn't put in the original issue #9 is that imagined the truthiness to be configurable. I think my original thoughts were that it makes template logic more robust to enforce booleans in if statements. Although I don't have any real data to back this up, if you think this is what most people will want then I'm happy to make this the default.

I would agree that "strict" conditional evaluation can be a good thing, but the merit of that seems a bit lost on upon because it lacks builtin filters for these comparisons (making the convenience-correctness tradeoff not worth it, in my opinion).

Maybe we should take this opportunity to investigate #6?

In the meantime, I think it's a good idea to have this be a configurable setting, whichever the default may be. Off the top of my head, there could be a BooleanEval enum with Strict, Truthy, and Custom(Box<dyn Fn(expr, span) -> Result<bool, Error>>) that the user can set on the Engine? Or should this be another template-specific setting (#15)?

[rossmacarthur]

In the meantime, I think it's a good idea to have this be a configurable setting

I think we can discuss the in a separate issue #19 . I think let's get the build passing on this change and then we can merge it like it is now.

[rossmacarthur]

@lunabunn Needed this today, so I fixed the build and I'm going to merge if that's okay