Change return values to singleton strings

[soronpo]

If the string values returned will be singleton values (<: String with Singleton), then it's possible to perform more string operations at compile-time (using singleton-ops), and possibly even enforce naming conventions. E.g.:

def foo(implicit n: sourcecode.Name.Aux[N], check: RequireMsg[Substring[N, 3]=="Fun", "Name must start with Fun"] : Unit = {}

val Fun1 = foo //OK
val fun2 = foo //Error: "Name must start with Fun"