Write full list of tbl_pd methods

[NickCH-K]

I've started writing tbl_pd methods for dplyr verbs so that pdeclare status is preserved when using non-pmdplyr functions, and pdeclare status is dropped if the .i or .t variables are removed. The goal is to eventually have the full list of dplyr verbs covered, as well as as many other methods as possible. So the to-do is:

1. Write tbl_pd methods for all dplyr verbs
2. Figure out which non-dplyr functions should also have methods
3. Write methods for all of those too.

[NickCH-K]

83e2a40 fills in the major dplyr verbs, other than the scoped variants. Next up are the scoped variants, and rewriting what's there so it doesn't use :::

[NickCH-K]

108e3c6 includes all scoped variants and all major dplyr verbs except bind_rows which for some reason refuses to call bind_rows.tbl_pd (perhaps because it allows a list first argument?), also fixes other minor issues.