Allow non-Sendable construction of UnxipStream.xip

DataReader (and potentially some of the bare sequences that people would
want to use here) are intentionally not Sendable. Unxip.makeStream
consumes them, but the UnxipStream.xip factory method takes them to set
up generic parameters correctly. This conflicts with Sendable checking
because the compiler thinks that the first call could stash away a
reference, preventing it from being sent into unxip's isolation domain.
However, these methods don't actually *do* anything with the parameter:
we solely use it for its type. Serendipitously taking the parameter as
@autoclosure (which we have no need to evaluate) is enough to satisfy
the compiler that we have no interest in the parameter's value.

Also, clean up some code I split up earlier and accidentally committed.

unxip.swift

@@ -1319,11 +1319,11 @@ public enum Disk: StreamAperture {
 }
 
 public struct UnxipStream<T: StreamAperture> {
-	public static func xip<S: AsyncSequence>(wrapping: S) -> UnxipStream<XIP<S>> where S.Element: RandomAccessCollection, S.Element.Element == UInt8 {
+	public static func xip<S: AsyncSequence>(wrapping: @autoclosure () -> S) -> UnxipStream<XIP<S>> where S.Element: RandomAccessCollection, S.Element.Element == UInt8 {
 		return .init()
 	}
 
-	public static func xip<S: AsyncSequence>(input: DataReader<S>) -> UnxipStream<XIP<S>> where S.Element: RandomAccessCollection, S.Element.Element == UInt8 {
+	public static func xip<S: AsyncSequence>(input: @autoclosure () -> DataReader<S>) -> UnxipStream<XIP<S>> where S.Element: RandomAccessCollection, S.Element.Element == UInt8 {
 		return .init()
 	}
 
@@ -1346,8 +1346,7 @@ public struct Unxip {
 	}
 
 	public static func makeStream<Start: StreamAperture, End: StreamAperture>(from start: UnxipStream<Start>, to end: UnxipStream<End>, input: sending Start.Input, _ option1: Start.Options? = nil, _ option2: Start.Next.Options? = nil) -> End.Input where Start.Next.Next == End {
-		let input = Start.transform(input, options: option1)
-		return Start.Next.transform(input, options: option2)
+		return Start.Next.transform(Start.transform(input, options: option1), options: option2)
 	}
 
 	public static func makeStream<Start: StreamAperture, End: StreamAperture>(from start: UnxipStream<Start>, to end: UnxipStream<End>, input: sending Start.Input, _ option1: Start.Options? = nil) -> End.Input where Start.Next == End {