[swift-evolution] [Pitch] Add an all algorithm to Sequence

[Robert Bennett]

I'm don't think we need all(equal:).
1) For a host of reasons, having a single signature for a function name is better than having multiple signatures when the single signature is capable enough.
2) A list containing a single distinct element is not a special enough case to check for to warrant its own function signature.

all(equal:) can be replicated easily enough with nums.all { $0 == 9 }. Unlike all(equals:), this is extendible to non-Equatable types with equatable members.