Change ToPrimitive() (to_primitive()) hint to be an enum, instead of string

[HalidOdat]

What's wrong with the current implementation of to_primitive?
The problem is that we use a string (&str) as the hint, so when we use to_primitive we always have to do a string comparison, which is not efficient.

To fix this we need to create a new enum PreferredType which would contain String, Number and Default field and use it to compare it instead of &str

More information:

• ECMAScript specification
• Boa to_primitive implementation