Add a way of providing information about borrows in external types

[sonmarcho]

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External types are opaque, but we need to know if some of their lifetimes are used in mutable borrows or not to properly generate the backward functions.

Example :

struct S<'a, T>;

impl<'a, T> S<'a, T> {
  fn use(self);
}

If S doesn't use 'a for mutable borrows, then the pure model for use will have the following signature:

def use {T} (x : S T) -> Result () := ...

If it uses 'a for mutable borrows, it will have the following signature:

def use {T} (x : S T) -> Result (S T) := ...