'Cancel' for PromiseKit #3

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dougzilla32 wants to merge 3 commits intoPromiseKit:masterfrom

cprkit:CoreCancel

Commits on Oct 7, 2018

'Cancel' for PromiseKit -- provides the ability to cancel promises and promise chains
dougzilla32
committed

Commits on Oct 9, 2018

'Cancel' for PromiseKit -- fix HomeKit CI build (was missing the "#ifdef !os(tvOS) && !os(watchOS)")
dougzilla32
committed

Commits on Oct 16, 2018

'Cancel' for PromiseKit -- remove cancellable wrappers (they are unnecessary)
dougzilla32
committed