Draft: Reduce memory usage of Json

[nordlow]

This reduces Json.sizeof from 64 to 40 bytes.

The memory usage could be reduced further by making use of an Algebraic data type or another more efficient json library.

Optionally, a Json node could be represented using a dual-pointer; one pointing to the tree and one to the current node. That would enable efficient storage of the filename inside the tree part.

For even denser tree representations, mir-ion provides a solution.

The m_fileName is only used in three diagnostics calls to enforceJson. Need a decision about whether we should drop that field or not. If it should be kept we need to find a way to propagate it without storing it in the node. That's trivial to solve except for Node.opIndex and Node.opCast.