engine: ast: rework the item Use

[W95Psp]

Currently, we export in the AST an item Use.
In Rust, [pub[(visibility)]] use some::path is twofold:

1. it allows to bring something in scope, possibly under a new name, e.g. use some::path as name or use some::path::*
2. it allows a module to re-export names

Importantly, rustc resolves names very early: when we observe the THIR, such renamings are all gone. Thus, when a module module has a pub use some::path::to::foo as bar, we will never see any reference to module::bar: instead we will see some::path::to::foo.

In consequence, the (2) is not something we are concerned by: we never want to re-export names that were re-exported in Rust by a use item.

However, in the backends, we may be interested in (1): it may be useful to have --as hints-- that the Rust developer wanted to open a scope or to rename an item. We can use this information locally to improve the quality of the code we generate.
Note that this is purely cosmetic: we can always use the full paths, which is what we do in the current F* printer, but that may change.

Next steps

We discussed with @cmester0 and @maximebuyse about this (while discussing #1030).

In (1), use items are not items properly speaking, just module-wise hints. More precisely, we don't want to give such use hints any name.
Thus, I think we should introduce a notion of module in the engine (maybe as shallow items), and carry a list of use hints there.
This way the backends can pick up this information. We anyway need a place to store comments and attributes on modules, so this design looks good to me.