Macro lucet_runtime_internals::with_ffi_arcs

macro_rules! with_ffi_arcs {
    ( [ $name:ident : dyn $ty:ident ], $body:block ) => { ... };
    ( [ $name:ident : $ty:ty ], $body:block ) => { ... };
    ( [ $name:ident : dyn $ty:ident, $($tail:tt)* ], $body:block ) => { ... };
    ( [ $name:ident : $ty:ty, $($tail:tt)* ], $body:block ) => { ... };
}

Wrap up the management of Arcs that go across the FFI boundary.

Trait objects must be wrapped in two Arcs in order to yield a thin pointer.