Add TEnum* TEnum::GetEnum(const char* OR std::type_info)

minimising the string operations.
The chain of operations to get to the enum are:
1) Get the type info name
2) Demangle it in a portable manner
3) Chop the name into scope name and enum name
4) Use the ROOT meta interface to get the TEnum
The implementation is not the most performant of all indeed
one passes always via string manipulation.
To minimise the impact of it C low level routines were used.