It replaces `(Linkage, Visibility)`, making the code nicer. Plus the
next commit will add another field.

This means we call `MonoItem::size_estimate` (which involves a query)
less often: just once per mono item, and then once more per inline item
placement. After that we can reuse the stored value as necessary. This
means `CodegenUnit::compute_size_estimate` is cheaper.

They're quite rare, and ignoring them simplifies things quite a bit, and
further reduces the number of calls to `MonoItem::size_estimate` to the
number of placed items (one per root item, and one or more per reachable
inlined item).

It doesn't seem worthwhile now that `MonoItem::size_estimate` is called
much less often.

…s, r=<try>

Streamline size estimates

Makes things nicer and a tiny bit faster.

r? `@wesleywiser`

This allows the compiler to cache the result.

This also moves the field-level mut restriction to a single location. It
attempts to resolve the missing errors for when a restricted trait is
implemented outside the defining crate, but the compiler does not
currently store information for other crates.

This allows for static verification that the wrong restriction is never
provided.

- Make diagnostics more translatable
- Avoid ICE on potential future bug
- Use `#![crate_type = lib]`