dirstate-tree: Serialize to disk The existing `pack_dirstate` function relies on implementation details of `DirstateMap`, so extract some parts of it as separate functions for us in the tree-based `DirstateMap`. The `bytes-cast` crate is updated to a version that has an `as_bytes` method, not just `from_bytes`: https://docs.rs/bytes-cast/0.2.0/bytes_cast/trait.BytesCast.html#method.as_bytes Drive-by refactor `clear_ambiguous_times` which does part of the same thing. Differential Revision: https://phab.mercurial-scm.org/D10486