87261 rust lines added 76766 / 77063 C++ lines removed ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓░ 99 %

#riir #rustlang

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The reader and input stack have been ported, which is basically everything. There's still some entry points in C++ (PR being reviewed) and test helper binary (might make a good external contribution as it's entirely self-contained), but almost all of the C++ is gone, and with it large chunks of the FFI.

Now we just have the second 90% to go - making sure this rewritten fish is portable and distributable!