rust-lib: move `Dirstate*Error` to the `dirstate` module That's where they belong and should always have been there.

rust-lib: only export very common types to the top of the crate This was done very early in the Rust project's lifecycle and I had very little Rust experience. Let's keep the `DirstateParents` since they'll pop up in all higher-level code and make the rest more explicit imports to make the imports less confusing and the lib less cluttered.

rust-dirstate: remove star exports This makes the crate's imports confusing and muddies the discovery of the code.

rust-dirstate: merge `dirstate_tree` module into `dirstate` The historical reasonning for `dirstate_tree` existing in the first place is that a new approach was needed for the tree-like dirstate and it was easier to start somewhat fresh. Now that the former dirstate is (long) gone, we can merge those two modules to avoid the confusion that even the module creators sometimes get.

rust-revlog: rename `end` to `data_end` This is more explicit.

rust-revlog: rename `start` to `data_start` This is more explicit.

rust-revlog: rename `length` to `data_compressed_length` This is more explicit and will not be confused with `len`.