Prevent underflow when using a MockI2CDevice with a RegisterMap.offset = 0x0

…re i2c.

Co-authored-by: Nick Stevens <nickastevens83@gmail.com>

Fulfillment of the I2c trait contract

Prepare 0.6.1 release

macOS 11 runners have been deprecated! Need this for CI to pass.

This change enables users of `i2cdev` to create generic functions on `T:
I2CTransfer` that have output buffers constructed separately from the
`I2CMessage` array, like the following:

```rust
fn smbus_read_post_box<T>(i2c: &mut T, offset: u16, out: &mut [u8])
	where for<'a> T: I2CTransfer<'a>,
{
	let addr = offset.to_be_bytes();
	let mut messages = [
		T::Message::write(addr),
		T::Message::read(out),
	];
	i2c.transfer(&mut messages).expect("uh oh");
}
```

Before this, `messages` would not satisfy the constraints of `.transfer()`,
because `messages` does not live as long as one of the output buffers `out`:

```
error[E0597]: `messages` does not live long enough
  --> src/smbpbisensor.rs:69:19
   |
63 |     let mut messages = [
   |         ------------ binding `messages` declared here
...
69 |         .transfer(&mut messages)
   |                   ^^^^^^^^^^^^^ borrowed value does not live long enough
...
78 | }
   | -
   | |
   | `messages` dropped here while still borrowed
   | borrow might be used here, when `messages` is dropped and runs the destructor for type `[<T as I2CTransfer<'_>>::Message; 2]`
```

The error message is a little confusing, but basically `&'a mut [Self::Message]`
is forcing the array of `I2CMessage`s to match the lifetime of the buffers in
the messages, which is not strictly necessary: the array of messages can have a
different lifetime than the buffers. After this change, the above example
compiles successfully.

core: Relax lifetime constraint on `msgs` in `I2CTransfer::transfer()`

Derive common traits for public types + warnings fixes