Deprecate legacy APIs before we delete them in 1.0

[wprzytula]

There are a number of API items that we consider legacy, but still haven't labeled with #[deprecated] attributes. Let's do that in 0.16, so that in 1.0 we can remove them.

This is going to be a list of such items:

• LegacySerializedValues,
• TODO

[Lorak-mmk]

Actually let's do that in 0.15.1. It's possible we won't release 0.16 because of time constraints, especially if we want to release release candidate for 1.0