Method capacity

Source/external/fast_float.h:1900–1902

	return length == 0;
  }
  constexpr size_t capacity() const noexcept {
	return size;
  }
  // append item to vector, without bounds checking
  FASTFLOAT_CONSTEXPR14 void push_unchecked(limb value) noexcept {
	data[length] = value;