Refactor serializer to not rely on serde_json::Value

[radu-matei]

The quickest way to build a lexicographically sorted object was to use the serde_json::Value serializer, then ensure it doesn't contain any non-permitted values for canonical JSON.

Doing so, however, means we visit each node multiple times - which is strongly undesirable, and fixing this is the top priority.

The fix consists of completely reworking the serialization of maps (and by extension of all structures, since, for the sake of a JSON serializer, a structure is merely a map) to be represented as a BTreeMap, which is by default sorted by key.