Implementing `impl From<SystemClock> for Hertz` is a footgun, as
SystemClock is not Copy, so the automatic conversion consumes the
owned clock.
This is visible in the example i2c.rs:
```
let mut i2c = hal::I2C::i2c1(
pac.I2C1,
sda_pin,
scl_pin, // Try `not_an_scl_pin` here
400.kHz(),
&mut pac.RESETS,
clocks.peripheral_clock,
);
```
If the user wants to use both `i2c0` and `i2c1` at the same time,
copying from this example won't work:
```
error[E0382]: use of moved value: `clocks.peripheral_clock`
--> rp2040-hal/examples/i2c.rs:106:9
|
97 | clocks.peripheral_clock,
| ----------------------- value moved here
...
106 | clocks.peripheral_clock,
| ^^^^^^^^^^^^^^^^^^^^^^^ value used here after move
|
= note: move occurs because `clocks.peripheral_clock` has type
`PeripheralClock`, which does not implement the `Copy` trait
```
As getting the frequency from a clock doesn't really need ownership,
changing it to `impl From<&SystemClock> for Hertz` is both more
logical and provides better usability.
This is, however, a breaking change: Code relying on this trait
implementation needs to be changed by adding a `&`.
