Build Rust crate with Nix via crane sub-flake

Move Rust build into its own flake at rust/flake.nix using crane, and
reference it as an input from the top-level flake. The Haskell build
gets the Rust artifacts (header + static lib) via a haskell.nix
override that symlinks them into place before configure.

Use Cabal's Cgarnet_rs naming convention for extra-bundled-libraries
to satisfy the shared library install phase naming requirements.

Also extract inputs.nix, default.nix, and shell.nix for flake-compat
(nix-build / nix-shell) support.

project.nix

@@ -1,5 +1,5 @@
 { inputs ? (import ./inputs.nix)
-, garnet-rs ? inputs.garnet-rs.packages.${system}.default
+, garnet-rs ? null
 }:
 
 {