"Standardize" name for variables of type InterpCx

[Mandragorian]

When I made my first contribution to the main miri code base, I was for a brief moment (slightly) confused about the variables this and ecx, mostly used to store the result of eval_context_{ref,mut}. It would be nice if the name was consistent.

I see that this is most commonly used:

$ grep -r "this: " | wc -l
62
$ grep -r "let this = " | wc -l
341

$ grep -r "ecx: " | wc -l
107
$ grep -r "let ecx = " | wc -l
8

However, sometimes this is used to refer to other values, for example:

borrow_tracker/tree_borrows/tree.rs:        this: &mut TreeVisitor<'_>,

So personally I would propose using ecx to store the interpreter context.

Obviously this is not really important, and adds no value to the users, but if you agree that this might improve the code base, I am glad to send a PR.

[RalfJung]

We generally use this when we'd like to use self but cannot. So that's for all the methods in a EvalContextExt impl. You're not confused that self has many different types throughout the codebsae, so you shouldn't expect that from this either.

However. some of the ecx should indeed be this, e.g. in src/alloc_addresses/mod.rs. Basically all cases of let ecx = self.eval_context_ref/let ecx = self.eval_context_mut should be changed to this. But the ecx: ... should stay the way they are, IMO.