Avoid iterating Vec<T> when `ToProxy::Output == Vec<T>`

[chenyan2002]

The ToProxy trait iterates through Vec<T> to convert each element in the vector. When Output type is exactly Vec<T>, we can just return self to save time. Due to Rust's trait specialization, this is not possible at the moment.

There are two ways to do this: 1) Use unstable rust to build proxy component; 2) Implement ToProxy for all vector types appeared in WIT in the codegen, without using the generic T type.