wrong length detected for atomic<uint64_t>

[tamboril]

For some reason, a uint64_t is formatted fine, but an std::atomic<uint64_t>, which defines
operator uint64() gets truncated to an int.

So these yield different output:

auto c = std::atomic<uint64_t> { 0x0000'0001'0000'0021ull };
cout << fmt::format("{}", c) << endl;
cout << fmt::format("{}", c.load()) << endl;

Output: 
33
4294967329

[vitaut]

Fixed in 07ed421. Thanks for the great bug report!