Support dynamic return values

[dmvk]

Hi,

thanks for an amazing job you guys are doing! It would be really useful if Call return values could be changed dynamically as referred here:

#34 (comment)

Thanks,
D.

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[dmvk]

I signed it!

[googlebot]

CLAs look good, thanks!

[sivakku]

Any reason why this fix is not yet merged in? other wise calling Return inside Do() seems broken

[balshetzer]

Thanks for this PR. Another solution to this problem has been pulled with #126.