"warning: the item `CertificateChain` is imported redundantly"
"warning: the item `SessionId` is imported redundantly"

"warning: you are explicitly cloning with `.map()`"

"warning: the item `rustls` is imported redundantly"

This fixes a wedge of instances of:

```
warning: the item `String` is imported redundantly
  --> rustls/src/msgs/handshake.rs:27:5
   |
27 | use alloc::string::String;
   |     ^^^^^^^^^^^^^^^^^^^^^
```

Where `String` is present from the std prelude when built
for testing.  Like we just did in webpki, _always_ opt-in
to no_std, and then import the std prelude in tests where
necessary.

This is a complete mock for `rustls::client::ClientSessionStore`,
but we don't have tests that use 100% of its abilities.