rust-revlog: teach the revlog opening code to read the repo options This will become necessary as we start writing revlog data from Rust.

rust-index: pass data down to the Rust index This will allow us to start keeping the Rust index synchronized with the cindex as we gradually implement more and more methods in Rust. This will eventually be removed.

rust-index: add append method This is the first time the Rust index has any notion of mutability. This will be used in a future patch from Python, to start synchronizing the Rust index and the C index.

rust-index: add an abstraction to support bytes added at runtimes In order to support appending data to the Rust index, we need to abstract data access away from the immutable (on-disk) bytes, to seemlessly fetch either from the preexisting data or from the newly added data.

rust-mixed-index: move the mmap keepalive into a function The same code will be used for keeping the new index mmap around.

rust-mixed-index: rename variable to make the next change clearer We're going to add another mmap reference holder, so let's rename this one first.