The basic idea was to reduce the number of things dependent on the `Clock`
type. To that end, I replaced `Duration<Clock>` with `IDuration` typedef
for `std::nanoseconds`, and `FloatDuration<Clock>` with `FDuration`
typedef for `Duration<double, std::nano>`. We can generally assume that
any clock's duration can be expressed in nanoseconds, as long as we insert
`duration_cast`s into the right places.

Note that we cannot remove all dependence on `Clock` as a template
arguments, because functions that actually measure the elapsed time have
to use the Clock.

We also changed some template function arguments to pass plain function
pointers, so that the actual implementation can be placed into a cpp file.

We replaced some simple std::algorithm usage by loops, and reduced
header inclusion.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

This is kinda messy, because there is no good way to signal to
the compiler that some code uses direct comparison of floating
point numbers intentionally, so instead we have to use diagnostic
pragmas.

We also have to over-suppress the test files, because Clang (and
possibly GCC) still issue warnings from template instantiation
even if the instantion site is under warning suppression, so the
template definition has to be under warning suppression as well.

Closes #2406

With these changes, all these benchmarks
```cpp
BENCHMARK("Empty benchmark") {};
BENCHMARK("Throwing benchmark") {
    throw "just a plain literal, bleh";
};
BENCHMARK("Asserting benchmark") {
    REQUIRE(1 == 2);
};
BENCHMARK("FAIL'd benchmark") {
    FAIL("This benchmark only fails, nothing else");
};
```

report the respective failure and mark the outer `TEST_CASE` as
failed. Previously, the first two would not fail the `TEST_CASE`,
and the latter two would break xml reporter's formatting, because
`benchmarkFailed`, `benchmarkEnded` etc would not be be called
properly in failure cases.