Optimize type comparisons

[Razican]

Current situation
Currently, if the interpreter wants to know what type a value is, it has to call get_type(), which returns a &str, and compare it to another &str. This takes a pretty log time.

It would be much faster to just compare values in an enumeration, which should be at most 1-word size, and can therefore be stored in the stack easily.

We sill need the &str version of the type, though, since for example, the typeof operator needs to display it. I think the best option would be for the Type enum to implement a function called as_str() so that we can get the &str value if needed.

So, the idea is to have a Type enum, that is returned by the get_type() function here, and then, that enum to have an as_str() function with a similar code to the current function.

[Lan2u]

I'll give this a go