added some new type extension and operators

added some type extensions for System.String:
* MatchFromStart
* CommonPrefix
added some operators for Stringfy.Types.Stringfy:
* ++
added some operatos-skeletons for  Stringfy.Types.Stringfy:
* -?
* --?
* -
* --
added some operations for the type Stringfy.Types.Stringfy:
* Equals
* GetHashCode
* op_Equality
* op_Inequality
changed some type definitions