Fix: modified bit_length function to deal with inputting zero

Previously, bit_length(big_int<1, T>{0}), where T is some unsigned
integral type, caused Undefined Behavior.

Now, bit_length can properly deal with such input.
Note that we define the bit_length of zero to be 1.

Added <stdexcept> header to gcd.hpp

Fixed circular header dependency

Required standard is now C++20

Changed big_int definition, now uses unsigned-integral concept

Fixed warning in div function about division by zero

Small fix to place_at

Update README.md

Add note about not every function being constant time.
(Thanks @mratsim for asking about this in [issue 47](#47) )