dirstate-v2: Add support when Rust is not enabled This wires into `dirstatemap` the parser and serializer added in previous changesets. The memory representation is still the same, with a flat `dict` for `DirstateItem`s and another one for copy sources. Serialization always creates a new dirstate-v2 data file and does not support (when Rust is not enabled) appending to an existing one, since we don’t keep track of which tree nodes are new or modified. Instead the tree is reconstructed during serialization. Differential Revision: https://phab.mercurial-scm.org/D11520

dirstate-v2: Initial Python serializer This adds code seralizing a `map` and `copy_map` dicts into dirstate-v2 file formate. This is not used yet. Differential Revision: https://phab.mercurial-scm.org/D11519