Rust changes for v6.3

More core additions, getting closer to a point where the first Rust
modules can be upstreamed. The major ones being:

- Sync: new types 'Arc', 'ArcBorrow' and 'UniqueArc'.

- Types: new trait 'ForeignOwnable' and new type 'ScopeGuard'.

There is also a substantial removal in terms of lines:

- 'alloc' crate: remove the 'borrow' module (type 'Cow' and trait
  'ToOwned').
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