(and ‘expression’, if any, last)
This is a first step towards allowing expression arguments to become variadic

This seems to give about 15% speedup when compiling tests using GCC.

The tradeoff is that under certain circumstances, there is a chance for
false negative result, when the expression under test throws exception
and the test code catches it before it gets to the test runner.

Example:
``` cpp
TEST_CASE("False negative") {
try {
REQUIRE(throws() == "");
} catch (...) {}
}
```
This test case will succeed, reporting no assertions checked, instead of
failing as it would with `CATCH_CONFIG_FAST_COMPILE` disabled. However,
just removing the try-catch block inside client's code will fix this, so
it is worthwhile.

This change does not apply to CHECK* macros, because these are currently
specified as continuing on exception and thus need the local try-catch
to work as intended.

Works like FAIL, but does not abort test.
As proposed in #765

Info is not changed, intentionally.

Closes #639

- simpler
- less templates and machinery
- no cloning, copying or unnecessary heap allocations
- better factored