Add concurrency migration guide

Pod won't stop spitting up on the social URL

Remove remaining pre-Swift 4 syntax as deprecated in Xcode 12.5

Universal framework build

Silence Xcode update warnings

Merge pull request #308 from bignerdranch/zwaldowski/install-guide-late-2020

Update installation guide for Swift Packages

Make failure state more clear when using fallback playgrounds impl

Update CI configuration

Quiet Xcode 12.0 upgrade warnings

eraseTo conveniences

Remove isCancelled as Task requirement

Tests for Task.and(...)

Add Task.and(...) and overloads

Refactor the mapping logic out of Task.ignored()