add `joyn()` and `joyn_workhorse`

[zander-prinsloo]

The addition of

a) joyn_workhorse()

This function always does a full join, and uses match_type = "1:1" by default. If no by argument is specified, uses the intersection of column names. By default, sets argument suffix = c(".x", ".y"). This function executes a collapse::join(how = "full") unless match_type = "m:m", in which cases it executes data.table::merge.data.table(all = T, allow.cartesian = T).

b) joyn()

This function becomes the higher-level workhorse function of the joyn package, executing all the relevant checks. It no longer relies necessarily converts to data.table objects.

[randrescastaneda]

All tests passed but vignettes are not being built.