Add string fuzzer.

Author: treeform

tests/fuzz_strings.nim

@@ -0,0 +1,23 @@
+import random, jsony, unicode
+
+randomize()
+
+# ASCII
+for i in 0 ..< 100_000:
+  var s = ""
+  for i in 0 ..< rand(0 .. 100):
+    s.add char(rand(0 .. 128))
+
+  if s.toJson().fromJson(string) != s:
+    echo "some thing wrong!"
+    echo repr(s)
+
+# UNICODE
+for i in 0 ..< 100_000:
+  var s = ""
+  for i in 0 ..< rand(0 .. 100):
+    s.add $Rune(rand(0 .. 0x10FFFF))
+
+  if s.toJson().fromJson(string) != s:
+    echo "some thing wrong!"
+    echo repr(s)