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

Sources/Voxelotl/Random/DarwinRandom.swift

@@ -0,0 +1,33 @@
+public struct DarwinRandom: RandomProvider {
+  public typealias Output = Int
+
+  public static var min: Int { 0x00000000 }
+  public static var max: Int { 0x7FFFFFFF }
+
+  private var state: Int
+
+  init() {
+    self.state = 0
+  }
+
+  public init(seed: Int) {
+    self.state = seed
+  }
+
+  mutating public func seed(with seed: Int) {
+    self.state = seed
+  }
+
+  mutating public func next() -> Int {
+    if self.state == 0 {
+      self.state = 123459876
+    }
+    let hi = self.state / 127773
+    let lo = self.state - hi * 127773
+    self.state = 16807 * lo - 2836 * hi
+    if self.state < 0 {
+      self.state += Self.max
+    }
+    return self.state % (Self.max + 1)
+  }
+}