Add test for string_ref::operator ==

Add test for string_ref::operator !=
Add test for string_ref::rfind
Add test for string_ref::find_last_of
Add any::clear member function per most recent update to any draft

Add test for any::clear
Fix core::any_cast to follow the newest changes of the proposal
Fix string_ref::find_first_not_of

Fix string_ref::find_last_not_of

Add test for string_ref::find_last_not_of
Add test for string_ref::remove_prefix

Add test for string_ref::remove_suffix
Add test for std::hash<string_ref>