There are 2 kinds of JSON marshaling:
1. Lossy raw marshaling, when type information is lost and
map keys are expected to be valid utf-8 strings.
2. Almost lossless marshaling, which can handle any non-recursive item.
Interop value preserves only type.
This commit implements the second.
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
