Add toString() methods for things so error messages are nice.

[erikrose]

Not everything has a toString() yet: rules in particular.

[aruna09]

Hey!! Could you elaborate more on this.

[erikrose]

When structures like rules complain about their input, they aren't specific enough to tell which rule they're talking about. To find examples, you can pretty much just grep the code for throw. Here are a few:

throw new Error(`The right-hand side of a dom() rule failed to specify a type. This means there is no way for its output to be used by later rules. All it specified was ${fact}.`);

throw new Error('The left-hand side of a rule() must start with dom(), type(), or and().');