Add RNG functionality to the math/ module.

The new RNG wraps up state for random number generation, so
one can have many rngs and even marshal and unmarshal them.
Adds math/rng, math/rng-uniform, and math/rng-int.

Also introduce `in` and change semantics for
indexing out of range. This commit enforces stricter
invariants on keys when indexing via a function call
on the data structure, or the new `in` function.

The `get` function is now more lax about keys, and will
not throw an error when a bad key is used for a data structure, instead
returning the default value.

test/suite7.janet

@@ -193,4 +193,19 @@
 # Trim empty string
 (assert (= "" (string/trim " ")) "string/trim regression")
 
+# RNGs
+
+(defn test-rng
+  [rng]
+  (assert (all identity (seq [i :range [0 1000]]
+                             (<= (math/rng-int rng i) i))) "math/rng-int test")
+  (assert (all identity (seq [i :range [0 1000]]
+    (def x (math/rng-uniform rng))
+    (and (>= x 0) (< x 1))))
+          "math/rng-uniform test"))
+
+(def seedrng (math/rng 123))
+(for i 0 75
+  (test-rng (math/rng (:int seedrng))))
+
 (end-suite)