Extract `GobLint.cartesian_*map` functions

[sim642]

~~This is on top of #1886 because I got the idea there.~~

GobList.cartesian_map unifies two kinds of patterns I found in various places:

1. List.map on BatList.cartesian_product.
2. List.map inside List.concat_map.

In a few places, the variants GobList.cartesian_filter_map and GobList.cartesian_concat_map are introduced.