Package rust and ocaml with nix

[ulrikstrid]

Usage:
nix build .#tuna to build OCaml parts
nix build .#vm_library to build rust parts

I'm seeing this when building thew vm_library:

failures:

---- get_balance stdout ----
thread 'get_balance' panicked at 'Lazy instance has previously been poisoned', /build/vm_library-1.0.0-vendor.tar.gz/once_cell/src/lib.rs:741:25
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- get_allowance stdout ----
thread 'get_allowance' panicked at 'Lazy instance has previously been poisoned', /build/vm_library-1.0.0-vendor.tar.gz/once_cell/src/lib.rs:741:25


failures:
    get_allowance
    get_balance

test result: FAILED. 3 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

error: test failed, to rerun pass '--test fa12'