Clean up the mess of mixed tabs & spaces in code files

[stakx]

... because it's really annoying when looking at code diffs (see e.g. #155). :)

This should wait until all other currently active PRs have been either merged or closed.

Seeing that many PRs are somewhat stale and need to be rebased anyway, this should happen as soon as possible, so that people making changes don't have to deal with this issue anymore.

[stakx]

Done in develop. For now, everything has been converted back to tabs, since this is what .editorconfig explicitly sets up and the majority of Moq's lines of code already use.