Sign conversion error in uuid_clock.hpp

[k3DW]

I got this error. It's causing Unordered tests to fail that weren't previously failing.

clang-linux.compile.c++ bin.v2/libs/unordered/test/visualization_tests.test/clang-linux-12/debug/x86_64/address-sanitizer-norecover/cxxstd-17-iso/stdlib-libc++/threading-multi/undefined-sanitizer-norecover/visibility-hidden/visualization_tests.o
In file included from libs/unordered/test/debuggability/visualization_tests.cpp:29:
In file included from ./boost/uuid/random_generator.hpp:10:
In file included from ./boost/uuid/basic_random_generator.hpp:10:
In file included from ./boost/uuid/uuid.hpp:9:
./boost/uuid/uuid_clock.hpp:66:34: error: implicit conversion changes signedness: 'std::chrono::duration<long, std::ratio<1, 10000000>>::rep' (aka 'long') to 'std::uint64_t' (aka 'unsigned long') [-Werror,-Wsign-conversion]
    return tp.time_since_epoch().count();
    ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
1 error generated.

I see a discrepancy in the uuid_clock class between int64_t and uint64_t. Not sure if one of these type should be changed to be the other one, or if static_cast is needed.

using rep = std::int64_t;
...
static time_point from_timestamp( std::uint64_t timestamp ) noexcept;
static std::uint64_t to_timestamp( time_point const& tp ) noexcept;

To reproduce it:

clang++ -Werror -Wsign-conversion path/to/boost/uuid/uuid_clock.hpp