Support long strings in Scala.js

[Raúl Piaggio]

When decoding a large string in Scala.js an exception is thrown RangeError: Maximum call stack size exceeded.

[Otto Chrons]

This is due to

boopickle/boopickle/js/src/main/scala/boopickle/StringCodec.scala

Line 187 in 864e89a

 js.Dynamic.global.String.fromCharCode.applyDynamic("apply")(null, cp.jsSlice()).asInstanceOf[String] 

passing the full character array to the function and browser JS engine not liking too long argument lists.

The solution is to break decoding of long strings into smaller parts and join the resulting strings later.