UniquePtr<CxxVector<T>> doesn't work for opaque C++ type T containing non-static borrows

[dtolnay]

There is a spurious 'static bound on the following UniquePtrTarget impl, which was inconsequential prior to #608, but now is quite limiting. We should find a way to avoid needing that bound.

cxx/src/unique_ptr.rs

Lines 236 to 238 in 3090721

	unsafe impl<T> UniquePtrTarget for CxxVector<T>
	where
	T: VectorElement + 'static,

Example:

use cxx::{CxxVector, UniquePtr};

#[cxx::bridge]
mod ffi {
    extern "C++" {
        include!("example/include/example.h");

        type Thing<'a>;
    }

    impl<'a> CxxVector<Thing<'a>> {}
}

fn f<'a>(_: UniquePtr<CxxVector<ffi::Thing<'a>>>) {}

error[E0477]: the type `Thing<'a>` does not fulfill the required lifetime
  --> src/main.rs:14:13
   |
14 | fn f<'a>(_: UniquePtr<CxxVector<ffi::Thing<'a>>>) {}
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: type must satisfy the static lifetime

[dtolnay]

Simply removing the static bound with no other code change:

error[E0310]: the parameter type `T` may not live long enough
   --> src/unique_ptr.rs:240:42
    |
236 | unsafe impl<T> UniquePtrTarget for CxxVector<T>
    |             - help: consider adding an explicit lifetime bound...: `T: 'static`
...
240 |     const __NAME: &'static dyn Display = &vector::TypeName::<T>::new();
    |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `TypeName<T>` will meet its required lifetime bounds