This means that code such as

```cpp
TEST_CASE() {
    SECTION("first") { SUCCEED(); }
    auto _ = GENERATE(1, 2);
    SECTION("second") { SUCCEED(); }
}
```

will run and report 3 assertions, 1 from section "first" and 2
from section "second". This also applies for greater and potentially
more confusing nesting, but fundamentally it is up to the user to
avoid overly complex and confusing nestings, just as with `SECTION`s.

The old behaviour of `GENERATE` as first thing in a `TEST_CASE`,
`GENERATE` not followed by a `SECTION`, etc etc should be unchanged.

Closes #1938

Closes #1954

* Successive executions of the same `GENERATE` macro (e.g. because
of a for loop) no longer lead to multiple nested generators.
* The same line can now contain multiple `GENERATE` macros without
issues.

Fixes #1913

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

Originally the tests were from #1912, but as it turned out, the issue
was somewhere else. Still, the inputs provided were interesting, so
they are now part of our test suite.

It did not clear out all of its internal state when switching from
one pattern to another, so when it should've escaped `,`, it took
its position from its position in the original user-provided string,
rather than its position in the current pattern.

Fixes #1905