Rename `DelayUs` to `BasicDelay`, add `delay_ns`.

[Dirbaio]

As discussed in today's meeting, we should add nanosecond precision to the delay trait.

Fixes #521 Fixes #535

Though as discussed in the meeting as well, we should add configuration settings to the SpiDevice impls in embedded-hal-bus to do the delays, and document drivers are discouraged from using Operation::DelayNs for CS-to-clocks delays. Opened #537 to track this

[adamgreig]

What makes it "Basic"? I still prefer DelayNs in line with having DelayUs before, assuming we save Delay for some future trait, but maybe it's too confusing since it provides all three methods?

I think of it as "delay with ns resolution", plus methods to delay for longer units which can trivially be done with more nanoseconds.

[MabezDev]

What makes it "Basic"? I still prefer DelayNs in line with having DelayUs before, assuming we save Delay for some future trait, but maybe it's too confusing since it provides all three methods?

I also thought about making this point, but I guess as an end user with no knowledge of these traits I wouldn't assume DelayNs would be able to do other resolution delays. It's a bit subjective, to be honest. Maybe we can trial it in an rc and see what implementors think?

[Dirbaio]

it's "basic" as "not using the fancy hypothetical future Duration type that will magically solve all our problems"... :see_no_evil:

I think BasicDelay makes more sense than DelayNs, because DelayNs implies we also have DelayUs, DelayMs, which we don't. Also it might throw off users looking for microsecond or millisecond delays?

No strong opinion from me, I'm fine with DelayNs too. If people think DelayNs is better I'll change it.

[MabezDev]

Just to throw some other names into the mix:

• DelayU32 - Rationale: All trait methods use a u32 instead of core::time::Duration which I guess more accurately describes why it's not just called Delay.
• ImpreciseDelay or LowResolutionDelay - Depending on the implementor, there can be different behaviours, particularly for delay_ns.

I have no strong opinions on any of the suggestions so far, I'd be happy with any of them.

[Dirbaio]

DelayU32

same as DelayNs, it implies we have other traits in the set like DelayU16 but we don't.

ImpreciseDelay or LowResolutionDelay

hmm I'm not sure, there's nothing inherently "imprecise" about it, it just rounds up if needed. The Delay trait with the magic rainbow unicorn Duration type will also likely be spec'd to round up if the hardware can't reach the requested precision, it won't be more precise.