Consider switching to portable PDB symbol files

[stakx]

This is a reminder (not likely to be relevant before mid-2018) to check whether we can make the switch back to portable PDBs. We reverted (in #443) to classic Windows PDBs because of #428.

As mentioned in that issue, the main showstopper appears to be TFS. Microsoft will supposedly have fixed the issue in the 2018 version of TFS (as mentioned here).

[stakx]

This cannot go further as long as AppVeyor doesn't support .snupkg. (While we could put portable PDBs in the main package, doing some still bears the risk of interfering with some users' tools if they're not perfectly up-to-date (e.g. VSTS), and it's safer putting them in a symbols package.)