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

Given that in the 2 or so years that matchers are thing nobody complained,
it seems that people do not actually write this sort of code, and the
possibility will be removed in v3. However, to avoid correctness bugs,
we will have to support this weird code in v2.

 - Overrides added
 - usages of push_back() replaced with emplace_back()
 - Loop variable made const-refernce
 - NULL replaced with nullptr
 - Names used in the declaration and definition unified
 - size() replaced with empty
 - Identical cases merged