[lucic71]

This fixes the default of unique_ptr<string[]>. Before this PR the translation did not use the default of string, now it does.

I added a new node in Rules IR called default. When cpp2rust encounters this node, it replaces with the default of the substituted type.

This also generates more idiomatic code:

-               arr: std::array::from_fn::<_, 3_usize, _>(|_| <i32>::default()).to_vec(),
+               arr: std::array::from_fn::<_, 3_usize, _>(|_| 0_i32).to_vec(),