Unable to use RpcStatusType enum

[makoven]

RpcStatusType is defined in jsonrpc.ts, but not in jsonrpc.js. So this code throws an error:

import { RpcStatusType } from "jsonrpc-lite";
...
switch (req.type) {
  case RpcStatusType.invalid:
...

[antongolub]

Moreover, if we'd try to use this enum with ts-jest it will also be undefined. (kulshekhar/ts-jest#281).

I guess, we should make enum const:

export const enum RpcStatusType {
  request = 'request',
  notification = 'notification',
  success = 'success',
  error = 'error',
  invalid = 'invalid',
}

[antongolub]

@junyuanqi, @zensh could anybody review the fix?

[zensh]

@antongolub Thank you very much, v2.1.0 released.