checkChoice / assertChoice messages with large set of choices

[antalk2]

The output of

x="a"; checkmate::assertChoice( x, paste0(LETTERS, 1:1000  ))

is both long and truncated in a way that the closing of the set and
the value of x (}, but is 'a'.) is not shown.

The following patch modifies set_collapse() to limit its result to 80 characters
by replacing the middle part with "...". (Added the .txt extension to make github
to accept the attachment)

0001-set_collapse-witdh-80L.patch.txt

• + This would make the messages affected more readable.
• - Adds package 'stringr' as a dependency.
  • This could be reduced.

The following patch (to be applied on top of the above) removes the dependency
on stringr. Includes a modified copy of stringr::str_trunc() (using nchar() and substr())

0002-include-modified-copy-of-stringr-str_trunc.patch.txt