Store map as externalizer for Swizzles

This document describes using a store map as an externalizer for Swizzles.

A store map is an externaliser for Swizzles.

By constructing an RStoreWriteStream and specifying the store map as an externalizer, the stream ID associated with a Swizzle can be externalised by applying the templated stream operator<< to the Swizzle.