construct_info() and info arg

[moodymudskipper]

should be used to record info when some events are encountered :

• state dependent issues: data frame and classed columns #166, tibbles and classed columns #155
• corrupted elements
• definition of pointers or environments using env() or external_pointer()
• singular things like, creating a data table and ignoring the pointer, or any scrubbing
• Use of extra packages (e.g. using tibble::tibble() requires the tibble package)

The different between info and issues is that whatever is described in info didn't prevent the output to be faithful, (1) when checked right after creation, (2) according to waldo::compare().

In fact we might even check if a compare_proxy method exists and inform in that case.

We might get an info = NA arg that works just like check.