Handle empty TPI and IPI streams

[jan-auer]

The ID stream may be completely empty if there are no ids in a PDB. Parsing the header fails in this case with an UnexpectedEof error, which is not clean. Instead, we can return an Option that explicitly indicates that there is no ID information available.

[jan-auer]

Alternatively, we could also create an empty object.

[jan-auer]

@willglynn I've pushed up an alternate implementation where IdInformation is not optional. Instead, it internally handles an empty stream. While implementing, I noticed that the shifted count was not rounded up properly. However, returning 0 as max_index for such streams seems like a bit of a hack.

[jan-auer]

Going with the second option now, as it seems a bit more ergonomic.