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.

src/core/cfuns.c

@@ -104,7 +104,7 @@ static JanetSlot do_debug(JanetFopts opts, JanetSlot *args) {
     janetc_emit(opts.compiler, JOP_SIGNAL | (2 << 24));
     return janetc_cslot(janet_wrap_nil());
 }
-static JanetSlot do_get(JanetFopts opts, JanetSlot *args) {
+static JanetSlot do_in(JanetFopts opts, JanetSlot *args) {
     return opreduce(opts, args, JOP_GET, janet_wrap_nil());
 }
 static JanetSlot do_put(JanetFopts opts, JanetSlot *args) {
@@ -275,7 +275,7 @@ static const JanetFunOptimizer optimizers[] = {
     {minarity2, do_apply},
     {maxarity1, do_yield},
     {fixarity2, do_resume},
-    {fixarity2, do_get},
+    {fixarity2, do_in},
     {fixarity3, do_put},
     {fixarity1, do_length},
     {NULL, do_add},