remove erroneous implicits

ToDatomicInj[DRef, DRef] simply isn’t needed and is covered in the more general ToDatomic type class
ToDatomic[DRef, DId] breaks the functional property of the ToDatomic as ToDatomic[DId, DId] holds as well, so DId could map to both DId and DRef!
refactoring implicits for writing to a reference attribute

introduce ToDRef type class
improve the PartialAddEntityWriter type class to properly handle writing to reference attributes
add tests for the implicits
optimize To/FromDatomic for DColl
tidy up Attribute2EntityReader implicits
Merge branch 'maintenance' into topic/implicit-conv

Pull in upstream changes.
minor: seal the ToDRef and DId traits
minor: fix use of DRef (should use DKeyword)
support singleton writes to set-valued ref attributes

the value for a cardinality-many, reference attribute can either be anything convertible to DRef or a traversable collection of anything convertible to DRef
Merge pull request #65 from pellucidanalytics/topic/implicit-conv

Fix implicit conversions to and from Datomic data