[plt-scheme] Cooperating with 3m

>From "Inside PLT MzScheme":
Garbage collection can occur during any call into MzScheme or its allocator, on anytime that MzScheme has control, except during functions that are documented otherwise. The predicate and accessor macros listed in section 2.1 never trigger a collection.

I just feel that scheme_utf8_decode and scheme_utf8_encode in "Chapter 11 String Encodings" will not trigger a collection, because they are pure C function. But it is not "mentioned otherwise" so I post here to make sure.

Thanks.

Sincerely,  
Chongkai Zhu