Import comparison gadgets from arkworks

[doc78]

Imported from arkworks:

• enforce_smaller_equal_than_le() gadget for Vec<Boolean> and modified previous implementation of enforce_in_field();
• conditionally_add and conditionally_select gadgets for UInt64;
• comparison gadgets for FpGadget;
• added conditionally_add function inside FieldGadget trait. and provided a default implementation for conditionally_add_constant;