+ Things have IDs
+ constructing Thing
+ adding (and noughtifying) metadata
+ implementing Relation, and Thing.push / Thing(a, b, c, ...)
+ copied Label over, as well as Thing.pair and Thing#find
+ fixing at() for empty metadata slots
(new API) [from µpaws] Copy over the basics of Thing

Squashes: 7a9ea..5c6b8, part of d5411
(new API) [from µpaws] Copy over Label, and tests for Thing#find

Squashes: part of d5411
(- fix) Make Thing#at() handle undefined members

Squashes: 76b96