Generic record as field value

[ilyakooo0]

Allow returning a generic record contains other such records, maps, lists and strings as a value for a metadata field for templating.

I thinks the semantics are obvious.

Would be useful for more advanced stuff. (I would find it useful :) )

(You can assign it to me.)

[bergus]

See #620 for a start :-)

I feel like the current Context implementation isn't really suited for this though. Currently it does represent "a field can extract a value from an Item", and this extraction happens in the Compiler monad. I haven't measured and am no expert, but this seems to be relatively inefficient when the data doesn't come from the Item, but is actually provided by the particular Context instance. A (data)structure that is meant to hold data seems more suitable for that.