Share memory between calls

[thedevbirb]

I tried to give a shot to #445 just for fun and as a challenge with this draft PR, which is still WIP.
This introduces a new struct called SharedMemory which is indeed shared between calls. I'd like a feedback on my implementation to see what it can done better.

Here are some implementation choices I made:

• its API matches almost completely the 'old' interpreter Memory implementation
• memory is allocated using this estimate https://2π.com/22/eth-max-mem/ (more on this below)
• it has a pointer current_slice which is internally used to refer to the portion of data reserved for the current context. This requires two unsafe methods get_current_slice and get_current_slice_mut which deferences the raw pointer
• current_slice pointer is updated when entering a new context (which is when a new Interpreter instance is created) using the use_new_memory method (every name is still WIP too) and when exiting it, which happens when the return_value method of Interpreter is called
• in order to make SharedMemory work properly, it is wrapped in a Rc for dealing with recursive calls and with an RefCell to allow mutability

Little problem: now test passes because I always allocate u32::MAX bytes. If I use the estimation however they fail because some tests have an impressive gas limit (at least 4_503_599_627_349_304), and my system fails to allocate so much memory (34_359_713_280 bytes). I don't really know how to handle this at the moment

Thanks in advance for any feedback!

[thedevbirb]

Closing this as I've found some stuff that needs more work than I thought