Rust 1.79 : LevelMasks could return Borrow

[tower120]

This rust-lang/rust#122055 should appear in Rust 1.79
We could try to return Borrow in LevelMasks instead of value.
This should allow to remove Clone constraint from BitBlock.

UPDATE. It turns out that we need more robust GAT support from Rust as well. for<'a> does not work in places where we need it.

[tower120]

implemented in 'SparseHierarchy' interface.