Implement *Assign ops

[adamgreig]

Implements +=, -=, *=, and /= where appropriate, by using the existing +, -, *, and / operators.

This allows for example:

let mut t = MillisDurationU64::millis(100);
t += MillisDurationU64::millis(900);

instead of what's currently required:

let mut t = MillisDurationU64::millis(100);
t = t + MillisDurationU64::millis(900);

Sorry if this has been considered before, I couldn't find any previous discussions. I tried to add it wherever it makes sense alongside an existing operator implementation.

[korken89]

Awesome, could you add one of each to the tests and update the CHANGELOG?

And thanks for adding! I had completely forgotten this!

[adamgreig]

Thanks! I've added tests (and a few extra tests for the non-assign versions where I couldn't find them, for Duration * u32, Duration / u32, Instant + Duration, Instant - Duration).

[korken89]

Looks good, thanks for fixing!

[korken89]

bors merge

[bors]

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