rust: id_pool: rename IdPool::new() to with_capacity()

rust: id_pool: rename IdPool::new() to with_capacity()

We want to change ::new() to take no parameters and produce a pool that
is as large as possible while also being inline because that is the
constructor that Rust Binder actually needs.

However, to avoid complications in examples, we still need the current
constructor. So rename it to with_capacity(), which is the idiomatic
Rust name for this kind constructor.

Reviewed-by: Burak Emir <bqe@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>