Let's add a concept of normalization: as preparation for signing json
records let's add a mechanism to bring JSON records into a well-defined
order so that we can safely validate JSON records.
This adds two booleans to each JsonVariant object: "sorted" and
"normalized". The latter indicates whether a variant is fully sorted
(i.e. all keys of objects listed in alphabetical order) recursively down
the tree. The former is a weaker property: it only checks whether the
keys of the object itself are sorted. All variants which are
"normalized" are also "sorted", but not vice versa.
The knowledge of the "sorted" property is then used to optimize
searching for keys in the variant by using bisection.
Both properties are determined at the moment the variants are allocated.
Since our objects are immutable this is safe.
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