String: Added four new methods to string.nit

Four new methods have been added to abstract class Text : is_upper, is_lower, to_snake_case, to_camel_case. The latter two relies on the two others in their implementation. Converting case may be useful for code generation, especially when wrapping extern classes in order to conform to snake case nit standard. 
Je trouve la contrainte sur le is_upper un peu arbitraire.

Si justement je veux transformer des chaines uppercase en snake_case je dois recoder l'algo? Domage...

Autant laisser le soin au client de faire le check:

    if not str.is_upper then str = str.to_snake_case