Method add

Source/external/fast_float.h:2385–2387

  }

  FASTFLOAT_CONSTEXPR20 bool add(limb y) noexcept {
	return small_add(vec, y);
  }

  // multiply as if by 2 raised to a power.
  FASTFLOAT_CONSTEXPR20 bool pow2(uint32_t exp) noexcept {