Prevent memory impl to access private fields + add missing pub/accessors

[nanocryk]

When trying to migrate frontier to the latest version, it seems impossible to implement the SubstrateStackMetadata because it cannot access private field StackSubstateMetadata.accessed.

This PR contains 2 changes :

• Fix visibility issues (accessor + pub Accessed)
• MemoryStackSubstate was "cheating" by accessing private fields of SubstrateStackMetadata since it is declared in a submodule. To make it "fair" (= make sure other implementations can access the same fields), I moved executor::stack::state into executor::memory, and moved the trait StackState which seemed to be in the wrong place.

[sorpaas]

Can you re-export necessary structs from memory instead? Putting memory under stack is because memory is "memory-only stack executor".

[nanocryk]

Not sure to understand which re-export you want me to change. Could you make a suggestion on a specific line ?

I moved memory outside of stack to better distinguish the stack executor itself from the memory-only (replaceable) implementation, and also to ensure it is indeed replaceable by preventing it to access private field.

If you want to keep memory under stack, I can move the stack executor itself into its own submodule to still prevent private field accesses. Do you a specific name in mind for such submodule ?

[sorpaas]

If you want to keep memory under stack, I can move the stack executor itself into its own submodule to still prevent private field accesses.

Yeah please do! Given that the name will probably not be exported I think the name of it doesn't matter much. My worst suggestion is to just name it impl, but if you have better names please use it!