Structured Member Decorations

[kvark]

Part of #5
Refines the decoration support with:

1. associating member decorations directly with the members
2. supporting multiple decorations

[antiagainst]

This one needs a rebase now. :)

[kvark]

Rebased now

[antiagainst]

I used BTreeSet initially to address the issue of potentially duplicated decorations. But I think that can be done with a pass if necessary. Using Vec should be more efficient. Nice! Thanks!