bug: ExecutionContext may not have both child_context and parent_context fields as Nullable<ExecutionContext>

[Eikix]

Bug Report

Currently, the ExecutionContext has both a parent_context and child_context field, in order to be able to:

• go up a context when a subcontext ends,
• access return data of a child context inside a parent context using returndatasize and returndatacopy

We cannot currently have two pointers pointing at the same struct.

Solution

Only have parent_context field and instead of child context, have a child_return_data: Option<Span>