Make memory and vector trait bounds publicly nameable

[dtolnay]

These trait bounds enable manipulating UniquePtr, SharedPtr, and CxxVector inside of generic code.

Example:

use cxx::memory::{UniquePtr, UniquePtrTarget};
use std::fmt::Display;

pub fn take_generic_ptr<T>(ptr: UniquePtr<T>)
where
    T: UniquePtrTarget + Display,
{
    println!("the unique_ptr points to: {}", *ptr);
}

Writing the same generic function without a UniquePtrTarget trait bound would not compile.

Closes #685.