Draft: more work on the simulations for the gas file

[clarus]

Currentyl, the simulations are not working yet. Here are some things that still need to be done:

• Simplify the LowM.t monad for links so that the errors are handled in a separate error monad. This should simplify some definitions, especially for the simulations that are built on top of it. In addition, we would be able to statically exclude the non-panic exceptions from the possible values returned by a function (break, continue, ...).
• Rename LowM.Let as LowM.LetAlloc to make it more explicit.
• We do this kind of LetAlloc for all the cases where we provide an explicit ty. There are such cases in the definition of the matching operators, that hence make one allocation too much. Fix it.
• Have a way to make all the definitions, rather automatically, using only as hypothesis that we have the "Stack.Access.t" property on all the references to the stack at call point.
• We that in hand, make a complete definition for the interpreter in the interpreter crate. We can admit all the loops for now, as well as not care about the aliasing. We will see how to properly isolate mutable references later.