Implement RegKey::encode_transacted

[dnlmlr]

• The RegKey::encode_transacted allows for encoding a serializable value using an existing transaction
• To make this work, a few changes were made to Encoder and Transaction
• Transaction now implements AsRef which shouldn't change anything
• Encoder now stores a generic AsRef instead of a fully owned Transaction
• This allows for two implementations, one is the existing one that owns the transaction and commits it on drop, the other only has a borrowed transaction that will not cause a commit on drop
• This way an existing transaction can be used
• Also added an example for transacted_serialization

[gentoo90]

Could you please add a test for this into tests/serialization.rs?

[dnlmlr]

I copied the existing 2 tests and changed them to use encode_transacted with a new transaction

[gentoo90]

Rebased onto winapi branch and released in versions 0.12.0 and 0.51.0