Move SRStatusCode typedef into header file

This is to easily enable developers to use an SRStatusCode value when closing a web socket.

The purpose of bc787cc is to be more conformant with [RFC 6455](https://tools.ietf.org/html/rfc6455), which states in [section 7.4.2](https://tools.ietf.org/html/rfc6455#section-7.4.2): `Status codes in the range 0-999 are not used.`