Padding

[Havvy]

Fixes #882

There are two initial commits. The first one does some general cleanup while the second actually defines padding. When reviewing, review each commit in order for better understanding.

You can see that while I'm still trying to describe padding as a property of a type, I have to actually say it's a property of a value for padding from fields. Thus I filed #934. Since Size and Alignment also are similarly contorted right now, I don't think figuring what to do about this should block this PR.