task promise use std::variant

[jbaldwin]

This is a step towards using a more modern container within the task promise and reduce some manual overhead around managing the unsigned char[] that would store the return value but retaining the benefits that that solution brought to the task's promise class.

Closes #196

[jbaldwin]

@a858438680 if you have some time I'd appreciate a review on this more modern take for the task optimizations.