Issue Description:

Trying to call this:

var string = String(count: 5, repeatedValue: "a")

results in this error:

<REPL>:1:14: error: could not find an overload for 'init' that accepts the supplied arguments

If I provide an explicit cast to Character for the repeatedValue: value, it works:

var string = String(count: 5, repeatedValue: "a" as Character)

A possible solution is to add “init(count:Int, repeatedValue s: String)” and remove existing API for UnicodeScalar and Character. This would be more powerful anyway.

NOTE: Requires a swift-evolution proposal.