clean up some random generator calls

Author: Azoy

stdlib/public/core/Collection.swift

@@ -929,12 +929,12 @@ extension Collection {
     using generator: inout T
   ) -> Element? {
     guard !isEmpty else { return nil }
-    let random = generator.next(upperBound: UInt(count))
-    let index = self.index(
+    let random = Int.random(in: 0 ..< count)
+    let idx = index(
       startIndex,
-      offsetBy: numericCast(random)
+      offsetBy: random
     )
-    return self[index]
+    return self[idx]
   }
 
   /// Returns a random element of the collection.

stdlib/public/core/CollectionAlgorithms.swift

@@ -445,16 +445,15 @@ extension MutableCollection where Self : RandomAccessCollection {
   public mutating func shuffle<T: RandomNumberGenerator>(
     using generator: inout T
   ) {
-    let count = self.count
     guard count > 1 else { return }
     var amount = count
     var currentIndex = startIndex
     while amount > 1 {
-      let random = generator.next(upperBound: UInt(amount))
+      let random = Int.random(in: 0 ..< amount)
       amount -= 1
       swapAt(
         currentIndex,
-        index(currentIndex, offsetBy: numericCast(random))
+        index(currentIndex, offsetBy: random)
       )
       formIndex(after: &currentIndex)
     }