One-shot mode now permits the sampling of differential pins, and the sampling of multiple pins simultaneously.
A new ChannelConfig structure has been introduced so that multiple channels can be configured individually. Further, the `sample` method now accepts a buffer into which samples are written.
Along the way, I've reset some default configuration to align with Nordic's settings in their nrfx saadc driver. Specifically, the channel gain defaults to 6 (from 4) and the time defaults to 10us (from 20us).
* Adds an executor for WASM runtimes based on wasm_bindgen.
* Add time driver based on JS time handling.
* Add example that can run in browser locally.
* Update to critical-section version that supports 'std' flag
Since `PeripheralMutex` is the only way to safely maintain state across interrupts, and it no longer allows setting the interrupt's priority, the priority changing isn't a concern.
This also prevents other causes of UB due to the interrupt being exposed during `with`, and allowing enabling the interrupt and setting its context to a bogus pointer.
`Peripheral` assumed that interrupts can't be preempted,
when they can be preempted by higher priority interrupts.
So I put the interrupt handler inside a critical section,
and also added checks for whether the state had been dropped
before the critical section was entered.
I also added a `'static` bound to `PeripheralState`,
since `Pin` only guarantees that the memory it directly references
will not be invalidated.
It doesn't guarantee that memory its pointee references also won't be invalidated.
There were already some implementations of `PeripheralState`
that weren't `'static`, though,
so I added an unsafe `PeripheralStateUnchecked` trait
and forwarded the `unsafe` to the constructors of the implementors.
