Do not use tree-based decoding for polymorphism

[qwwdfsad]

Currently, even for regular usages, Json operates in two modes -- streaming-based decoding and, in the case of polymorphism, JsonElement-based decoding.

The latter complicates maintainability, predictability of code execution and debuggability (e.g. printing proper error reporting, toString, code bloating and a separate-code path used for polymorphism-unrelated functionality).

My proposal is to completely get rid of tree-based deciding on the path of decodeFromString and decodeFromSteam and maintain it only for JsonElement serialization