rustdoc: summary line for hg_path_to_os_string The main motivation of this change is to avoid the TODO being the summary line, even though this leads to a pretty obvious summary. Then doc-comments for the other functions are introduced for consistency.

rustdoc: wording for checkexec Notably separating the summary line for correct display at module level.

rustdoc: fixed warnings about links This is the minimal fix making those that actually were supposed to be links to work (including in private items).

rust-changelog: introduce ChangelogEntry parent entries accessors Straightforwards now that lifetimes are explicit in `RevlogEntry` parent accessors.

rust-revlog: fix lifetime problem for RevlogEntry parent entries accessors Without this, the lifetime of the result is equated to the lifetime of the `self` reference, preventing callers, e.g., to take a `RevlogEntry` and return its `p1_entry()`, as it looks like returning something that does not outlive the *reference to* the `RevlogEntry`.

rust-revlog: explicit naming for `RevlogEntry` lifetime This matches what has been done in `revlog::changelog::ChangelogRevisionData`, and has the advantage of making things clearer when we introduce other, shorter lived lifetimes.

rust-changelog: introducing an intermediate `ChangelogEntry` Before this change, client code needing to extract, e.g, the Node ID and the description from a changeset had no other choice than calling both `entry_for_rev()` and `data_for_rev()`. This duplicates some (limited) computation, and more importantly imposes bad hygiene for client code: at some point of developement, the client code would have to pass over both entry and data in its internal layers, which at some point of development would raise the question whether they are consistent. We introduce the intermediate `ChangelogEntry` from which both conversion to the generic `RevlogEntry` and extraction of `ChangelogRevisionData` are possible. It might grow some convenience methods in the future. We keep the `data_for_rev()` method of `Changelog` for compatibility, pointing users at the more powerful alternative.

rust-changelog: added a test for `NULL_REVISION` special case The result is due to `Revlog.get_rev_data()` returning an empty byte string for `NULL_REVISION`, followed by special case for emtpty byte strings in `ChangelogRevisionData::new()`.