Add ">", "<", ">=", and "<=" to core library

[Nyhilo]

Added comparison operators to the full library as well as the is_neg function, which is a dependency of the comparison functions.

Also made a small change of if_num to is_num, to bring it into line with the other type-checking functions.

[zesterer]

That's a very clever hack! Just so you're aware, there are __less and __lesseq operators that are supported by the language, so these can be used to implement these operators.

[Nyhilo]

Oh nice. Without looking through the source code I just assumed all the internal operators were expressed in the core.at. This is good to know.

[zesterer]

core.at defines what it can, but there are certain intrinsic operators that the language simply can't function without. However, there aren't many of them.