Implementation of a stream-over-string class (a Readable with a string as input source) that can be used with Platformrecordmanager.

Used to supply SGML data from <literal> system identifiers.

The base construction is described eg. in the StackOverflow article linked to below; could use the following code instead

    var stringstream = new stream.Readable()
    stringstream._read = function noop() {}

but this is factored out into a separate class and also honors the Readable contract (eg. not pushing further data if previous push() returned false).