Added parsing from String for decoding Int64

Javascript isn't any good at handling large numbers natively. Instead,
any number over 53 bits is supposed to be stored as a string. In order
to support this, Argo should really add the ability to decode `Int64`
instances from `String`s, in addition to `Number`s.

ArgoTests/JSON/types.json

@@ -2,6 +2,7 @@
   "numerics": {
     "int": 5,
     "int64": 9007199254740992,
+    "int64_string": "1076543210012345678",
     "double": 3.4,
     "float": 1.1,
     "int_opt": 4