This means that bit-flag-like types with conversion to bool can be
asserted on, like so `REQUIRE(var & Flags::AddNewline)`.

This is a partial revert of b7b346c3e5603.

The old code caused warnings to fire under MSVC, and Clang <3.8.
I could not find a GCC version where it worked, but I assume that it
did at some point.

This new code causes all of MSVC, GCC, Clang, in current versions,
to emit signed/unsigned comparison warning in test like this:

```cpp
TEST_CASE() {
    int32_t i = -1;
    uint32_t j = 1;
    REQUIRE(i != j);
}
```

Where previously only MSVC would emit the warning.

Fixes #1880

Previously they could conflict with user-defined type traits that
were in the global namespace.

Fixes #1548

By default, it expands into a `static_assert` + `SUCCEED` pair, but
it can also be deferred to runtime by defining
`CATCH_CONFIG_RUNTIME_STATIC_REQUIRE`, which causes it to expand
into plain old `REQUIRE`.

Closes #1362
Closes #1356

The fix leaves an open question: should we keep treating refs
to static array of chars as strings, or should we instead
use `strnlen` to check if it is null-terminated within the buffer

Fixes #1238