Solves #6

This introduces a new suite of methods while maintaining the existing ones as a default behavior.

The new suite of methods and behaviors are:

• The Set type can be generically constructed with a custom shift indexing either through the set! macro or using the Set::new_with constructor.
• A new bittle::set_shr! has been introduces to explicitly build sets with shift-right indexing.
• Right-shift indexing can be explicitly used with a new family of methods where it's appropriate using names suffixed with *_shr.
• Generic shift indexing can be used with a new family of methods where it's appropriate using names suffixed with *_with. These take a generic argument which must be one of the following newly introduces types: DefaultShift, Shl (for shift-left indexing), or Shr (for shift-right indexing).