use arc4random to seed non-csprng, fleshes out random subsystem

Sources/Voxelotl/Random/RandomRange.swift

@@ -0,0 +1,20 @@
+public extension RandomProvider where Output: BinaryInteger {
+  mutating func next(in range: Range<Int>) -> Int {
+    range.lowerBound + self.next(in: range.upperBound - range.lowerBound)
+  }
+
+  mutating func next(in range: ClosedRange<Int>) -> Int {
+    range.lowerBound + self.next(in: range.upperBound - range.lowerBound + 1)
+  }
+
+  mutating func next(in bound: Int) -> Int {
+    assert(Self.min == 0)
+    assert(Self.max >= bound)
+    let threshold = Int(Self.max % Output(bound))
+    var result: Int
+    repeat {
+      result = Int(truncatingIfNeeded: self.next())
+    } while result < threshold
+    return result % bound
+  }
+}