A reagent reactive wrapper around Datascript's transactor

Although this is a rewrite of mpdairy/posh library it is much
more useful and simple. The problem with posh is that it tries
to do too much. It target both datomic and datascript. Therefore
it recreates the reagent/atom implementation internally.

This bring other problems with it:
- all queries must be cached (but reagent is capable of doing that
   with reagent/track)
- whenever the state changes, posh must figure out which query result
  should be updated (but reagent is capable of doing that by diffing
   the result of the queries)
- only the currently viewable components's queries should be executed
   (but reagent already does that since all inactive reagent/track are
   removed automatically)
- all the problems above also make posh a big library with hundreds of lines
  (reagent.tx has barely 50 even with all datascript functionality !!)

  See https://reagent-project.github.io/news/news060-alpha.html
  for more information