support for v3 and v4

[dherman]

Danny Yoo wrote:

Hi Dave,

I sent an earlier patch to struct.plt 2.3 to make it work with mzscheme 3.99, but didn't hear back from you yet. In any case, I took a look at it again, and the patch wasn't so good because it made it work only with mzscheme 3.99, which broke backwards compatibility.

Since you probably want to make it work regardless of version, I've updated the patch so that it uses version-case to do the appropriate version-specific stuff.

Summary of changes:

* I created a separate module 'private/struct-info-compat.ss' which
provides a function to get the predicate from the struct type syntax
object. I updated datatype.ss to use this library.

* As in the previous patch, I also modernized the test cases to use
SchemeUnit? 2. The tests involving hierarchy are failing, but I don't
think it's a problem with my patch, but rather with a missing
'definitions' collection that I don't know much about.

I hope this helps!