Add `Time::EpochConverter`?

[BigBoyBarney]

Hi!

I'm not sure whether this should go into the main crystal-db or the respective db drivers, but I recently discovered that Time::EpochConverter doesn't support from_rs, which makes sense, since it's an external shard.

I do consider this however quite useful, so I was wondering where this belongs.

[bcardiff]

Seems a reasonable addition to ./src/serializable.cr 👍