Mocking?

I'd be curious to hear thoughts from you guys on adding a mocking framework like [mockall](https://crates.io/crates/mockall) for PAC-generation?

Basically, I'd like to be able to `rp-pac { version = "0.1.0", features = ["mocking"] }` as a dev-dep, and have something like this in the generation (for example):

```rust
#[cfg(all(test, feature = "mocking"))]
use mockall:automock;
...
// in fieldset.rs
...
#[cfg_attr(all(test, feature = "mockall", automock))]
pub struct #name (pub #ty);

#[cfg_attr(all(test, feature = "mockall", automock))]
impl #name {
#items
}
...
```

Then, in my code I could use:
```rust
#[double]
use pac::PWM as PWM;
```

Which would automatically expand `PWM` to `MockPWM` when I'm in `test` -- this would let me mock registers etc. so that I can more easily test that the registers end up with the correct values (logic-wise, at least) in drivers without needing to involve a board.

Concrete example is working on [this PR](https://github.com/embassy-rs/embassy/pull/2855) where via mocks I could manually step through the PWM counter and verify that the correct div/top/compares/etc. are set for each step in a test.

Thoughts?