Lambda-accepting `UnsafeBufferOperations` need contracts for calls-in-place

[JakeWharton]

Kotlin 2.0 can see through inline funs now, but it does not infer calls-in-place.

One workaround for #360 would be to do

	override fun readAtMostTo(sink: Buffer, byteCount: Long): Long {
		val bytesRead: Int
		UnsafeBufferOperations.writeToTail(sink, 1) { bytes, startIndexInclusive, endIndexExclusive ->
			val read: Int = TODO()
			bytesRead = read
			read
		}
		return bytesRead.toLong()
	}

except without the contract the compiler complains bytesRead is not initialized at the return.