Rust

Remove usage of `extern crate`. #427

Job	Run time
build_on_wasm	33m 15s
build_with_vcpkg_installed_z3 (windows)	7m 15s
build	1m 3s
build_z3_statically (linux)	20m 7s
check-formatting	8s
build_z3_statically (macos)	12m 32s
run_clippy	1m 35s
build_z3_statically (windows)	34m 2s
	1h 49m 57s

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