implement pillar::format_glimpse() for rvars

[mjskay]

Currently pillar::glimpse() fails if a data frame has an rvar in it:

> library(posterior)
> data.frame(x = rvar(1:10))
              x
1 5.5 ± 3.02765
> pillar::glimpse(data.frame(x = rvar(1:10)))
Rows: 1
Columns: 1
Error: C stack usage  15922912 is too close to the limit

This can be fixed by implementing pillar::format_glimpse(), which should be pretty easy. Basically:

• if vector-like rvar, return character vector of elements converted to a display format. Some variant of format.rvar() should work
• if array-like (2 or more dims), return an abbreviation similar to what matrices / arrays do; e.g. pillar::format_glimpse(matrix(1:10)) returns "<matrix[10 x 1]>"