As I added more and more cases, I found the dir structure hard to
navigate. The new structure follows how you read a type:
e.g. map[string]*string -> map/string/ptr_string
This exposed some redundant cases and some missing cases, too. Now
map[string] is the "main" test case that exercises all the variants of
value types, and the other key types are just to prove that they
basically work.
