PwmPin: allow get_duty to return the old value for some time

[chrysn]

PWM is often implemented in a buffered way to allow glitch-free operation; as a result, implementing a strict "you get what you last set" is not feasible for some implementations.

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This change allows the implementation to decide which value to return for get_duty while it is being changed. The current wording is "current duty cycle", which seems to be quite clear but deviates from the usual semantics of getters and setters in a way that can lead to different interpretations.

In particular, the EFM32 implementation gives the old value until the cycle has actually been changed (at least since a semi-related WIP bugfix) -- but it seems to me that it makes sense to allow that given how it can be implemented, especially considering that basic embedded-hal traits should be widely implementable.

Alternatives:

• Prescribe "it's always the current duty cycle"
  • If that's the original intent, I'd be happy to change this PR to instead add "Note that setting a PWM duty cycle may not take effect immediately. After set_duty has been called, this reports the old duty cycle until the change has been executed." or similar.
  • I don't know whether it can be implemented like that everywhere.
• Prescribe "get after set gives last set value"
  • This may not be possible to implement on all platforms while keeping PwmPin zerosized
• Make set_duty block (or nonblockingly refuse access to the peripheral while it's being changed)

[rust-highfive]

r? @ryankurte

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[therealprof]

Makes sense to me.

[no111u3]

It's looks good for me, and I thinks it need to merge.

[eldruin]

LGTM.
@chrysn could you rebase this to master?