Fix problem defining arrays in class declarations (#2776)

- Fix `static_assert` failure using `int64_t` for map keys
- Add integration tests to verify before/after behaviour.
- Update documentation

For example, this code produces `__fstr__myIntArray not defined` error:

```
struct MyClass {
    DEFINE_FSTR_ARRAY_LOCAL(myIntArray, int, 1, 2, 3, 4, 5)
};
```

The same issue applies to maps and vectors.