There is an intermittent build failure and I suspect it's because the keys are too generic and are interned, and thus may have their refcount incremented or decremented by some other (background?) process.
Gets rid of properties defined as both MAKE_SEQ_PROPERTY/MAKE_MAP_PROPERTY, which are just a bad idea. Instead, adds a way for map properties to define a separate "keys" interface.
Fixes: #203
rdb