First non-trivial change for a while... because this involves reading from the OS, we should cache as much as we can in memory. So, group membership checking is done immediately after reading a group name, as the group structure has already been populated.
Currently there's only one numeric column, and that's the file size, so it gets
special treatment.
I was originally going to have a folder file size field be filled up with '-'s
as far as it could go, leaving it entirely up to the column how its field gets
formatted. But then I saw just one '-' working just fine, so I left it like
that. In the first try, columns could do anything they want when padding a
string (including changing the padding character or just changing it entirely),
but now there's no point.
Also, reverse the way columns are rendered: before, a column took a stat and a name to render; now, a file takes a column type to render. This means that most of the File data/methods can be private.
