SYSTEM_INFO does not implement Default

[Ben-Lichtman]

implementing Default would be useful since GetSystemInfo requires a SYSTEM_INFO out pointer. Without Default this is more difficult to produce.

[kennykerr]

This is a known limitation for some of the more interesting structs (mostly unions and structs containing unions). I plan to fix this. #656

[Ben-Lichtman]

Ah I didn't think of that, in retrospect that makes a lot of sense... I'll close this issue and just use MaybeUninit::zeroed().assume_init() as as stopgap for now.