Allow arbitrary record fields

[timothee-haudebourg]

Currently record fields (map keys) can only be text strings. This is consistent with JSON's data model, but not with many other data-model that allow arbitrary map keys, such as CBOR. There is no inherent limitation to TreeLDR's layout that would forbid non-string keys.

That being said, the only concrete syntax available to define TreeLDR layouts today is JSON. So even if we allow non-string keys, we would have no way of defining a layout accepting non-string keys with the JSON syntax.

[timothee-haudebourg]

I've added non-string keys in the tree data model. However there's still the JSON syntax issue. It could be solved by just using another syntax for layouts, like CBOR or RON.