Increase or remove `ensure` `Debug` output limit

[sirewix]

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