Allow to load db (with indexes) from map of reader.

I am serializing the database with `pr` and deserialize it with `cljs.reader/read-string` (eventually in IndexedDB and elsewhere with [konserve](https://github.com/ghubber/konserve) ). For this to work I cannot use the default `map->DB` anymore, since the index-sets are custom types in `0.1.5` and its construction methods are private (although cljs does not enforce `defn-` btw., so I could [hack around](https://github.com/ghubber/wefeedus/blob/master/src/cljs/wefeedus/core.cljs#L150) ). I am not sure whether this is your prefered way, but some public loading routine was nice. 
I have written an [eventual consistent p2p synching layer](https://github.com/ghubber/geschichte) for the last half year btw. and datascript fits nicely as the datalog component we planned originally btw. Thank you very much!