Move tibble to suggests and make as_tibble optional

[dshemetov]

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[dshemetov]

Closing this after a Slack discussion. The TL;DR is that it doesn't seem worth the effort.

The main goal here was to (1) slim down dependencies before release to make; (2) be wary of conflicting version requirements for R. However, there are complications as data.frame and tibble can behave differently for downstream calls (e.g. dropping dimensions when selecting one column). Since (1) is nice to have, but not a necessity, and tibble is popular, we can overlook it. For point (2), we chose to go with supporting the version of R used by the current Ubuntu LTS: in this case Ubuntu 20.04, LTS until 2025, which uses R 3.6.3 in r-base. Since tibble needs >3.4.0, we are fine.