operator-(const time_point &, const duration &) mishandles unsigned numbers

The binary subtraction operation `tp - d` is implemented as `tp + (-d)`.
Because the unary minus operator is used, the result is wrong if the underlying representation type is unsigned.

For instance :
```
#include
#include
#include
#include

// namespace chrono = std::chrono;
// using std::micro;
namespace chrono = boost::chrono;
using boost::micro;

int main() {

const auto tp = chrono::system_clock::time_point(chrono::milliseconds(0));
chrono::duration delay(1);
std::cout << (tp - delay).time_since_epoch().count() << std::endl;
assert((tp - delay).time_since_epoch().count() == -1000);
return 0;
}
```
Running this program produces the following output:
```
4294967295000
a: /tmp/a.cpp:16: int main(): Assertion `(tp - delay).time_since_epoch().count() == -1000' failed.
```

Note that the std::chrono from gcc does not exhibit this defect.

Locate the implementation of operator-(const time_point&, const duration&) in the Boost.Chrono sources and inspect how subtraction is expressed for unsigned representations. Add a regression test based on the uint32_t duration example, verifying that subtracting one microsecond-scaled unit produces the expected negative time point.

Written by the indexing model from the issue text.