explicitly drop box ptr

[sorhawell]

compiling extendr with latest rust nightly I get warnings for every #[extendr] macro invocation

warning: unused return value of `Box::<T>::from_raw` that must be used
     --> src/rdataframe/mod.rs:98:1
      |
   98 | #[extendr]
      | ^^^^^^^^^^
      |
      = note: `#[warn(unused_must_use)]` on by default
      = note: call `drop(from_raw(ptr))` if you intend to drop the `Box`
      = note: this warning originates in the attribute macro `extendr` (in Nightly builds, run with -Z macro-backtrace for more info)

the warnings come from this update, where it is required to explicitly drop boxed pointers

another contributer to rust-lang solved it like this:

I have used the same pattern in this PR

[multimeric]

I think rust-lang/rust#99270 is the original original source of this change. Anyway, looks good!

[Ilia-Kosenkov]

Ok, looks good!