Increase or remove `ensure` `Debug` output limit

Currently the limit is set to 40 bytes in #195, there are few issues with that:

1. Unpredictability. I discovered this trying to figure out why in some cases I don't get useful information during test failures, while in other cases I do. This is inconsistent
2. 40 bytes is only useful when comparing numbers, for real use cases that will very often be not enough. Something like 400 bytes would suffice I guess
3. The initial argument
> This avoids dumping enormous objects into the error message implicitly.

Does not make *that* much sense:
* `assert_eq` doesn't have that limitation
* `ensure` is used primarily in tests, having a bit of a spill there wouldn't hurt, while not getting useful information actually might hurt

So I wonder if this limit can be completely removed.

On the contrary, the difference with `assert_eq` is that with `ensure` the memory needs to be allocated, while with `assert_eq` it probably prints right away, so some limit probably makes sense for this reason