Migrate to portable PDB

[ForNeVeR]

When publishing the release 0.8.0, I've tried to publish it without PDB, but with an external symbol package.

It didn't go well: I've received the following email from NuGet after the publishing.

The symbol package WpfMath 0.8.0 failed validation because of the following reason(s):

• The uploaded symbols package contains one or more pdbs that are not portable.

Your symbol package was not published on NuGet Gallery and is not available for consumption. You can reupload your symbol package once you've fixed the issue with it.

I can remember about two possible reasons we use non-portable PDB (aka <DebugType>Full</DebugType>):

• something was wrong with portable PDBs and F# compiler with anonymous structures (we don't use the anonymous structures anymore)
• programmatic stack trace access was broken with portable PDBs, and ApprovalTests.NET require that to properly detect the test framework we use

We'll need to check that and somehow fix the issues.

[ForNeVeR]

Alternatively, we could just put the PDB back into the package (and enable SourceLink!). Please reconsider.