Lifecycle updates

[hadley]

Pre 1.0.0

• [x] funs() to defunct. Since 0.8.0 (Feb 2019).
• [x] select_vars() and friends to defunct. Since 0.8.4 (Oct 2019)

Probably low-risk 1.0.0

• [x] Move bench_tbls() to defunct. GitHub code search suggests no CRAN users.
• [x] location() to defunct. Can't easily search for, but likely to be few uses.

Increase warning frequency

And then maybe for everything else 1.0.0 and older we should switch to warning unconditionally?

• [ ] combine() to defunct? Hard to search for, but could be some usage.
• [ ] progress_estimated() -> deprecate_warn(). Make internal version that do() can continue to use.
• [ ] tbl_df() and as.tbl() still seem to have CRAN uses. Move to heavy warnings.
• [ ] group_keys() and group_indices() without arguments?