Refactor FFI variable type for Rust library #85

Job	Run time
busted (luajit-openresty, ubuntu-latest)	24s
busted (luajit-openresty, windows-latest)	1m 36s
busted (luajit-openresty, macos-latest)	1m 15s
busted (luajit-2.1.0-beta3, ubuntu-latest)	24s
busted (luajit-2.1.0-beta3, windows-latest)	1m 40s
busted (luajit-2.1.0-beta3, macos-latest)	55s
busted (luajit-git, ubuntu-latest)	23s
busted (luajit-git, windows-latest)	1m 43s
busted (luajit-git, macos-latest)	1m 29s
busted (5.4.6, ubuntu-latest)	24s
busted (5.4.6, windows-latest)	1m 19s
busted (5.4.6, macos-latest)	40s
busted (5.1.5, ubuntu-latest)	24s
busted (5.1.5, windows-latest)	1m 23s
busted (5.1.5, macos-latest)	1m 17s
busted (5.3.6, ubuntu-latest)	24s
busted (5.3.6, windows-latest)	1m 32s
busted (5.3.6, macos-latest)	1m 2s
busted (luajit-git, ubuntu-latest, rust)	26s
finish	3s
publish	0s
	18m 43s

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