1.2.0

New serialization: datascript.core/serializable & datascript.core/from-serializable. Prepares database for fast to/from JSON serialization.

In some benchmarks, CLJS version of datascript.serialize + JSON.stringify/.parse lead to 10x faster snapshot reads, 3x faster snapshot writes, at roughly the same snapshot size (maybe 10% smaller). All numbres compared https://github.com/tonsky/datascript-transit.