Add darkwallet ephemeral key serialization.

[evoskuil]

Darkwallet used a distinct parse format for stealth encoding. This may help people attempting to recover funds.

[evoskuil]

This is simple and more of an academic exercise, and stealth at this layer is pretty elegant. So holding on to this despite dropping server indexation of stealth payments in v4. I also used this once to salvage someone's btc from darkwallet limbo.