Add PureXML.Emitting.Writer, the libxml2 xmlTextWriter model: emit a document
with writeStartElement/writeAttribute/writeString/writeEndElement and friends,
never building a tree. Escaping is extracted into a shared Emitting.Escaping
helper used by both the writer and the serializer, so compact writer output is
byte-identical to serializing the equivalent tree. Pretty mode indents element
children (forward-only, so prefer compact for mixed content). Closes#32.
Reimplemented from the libxml2 model; no upstream code copied. 6 tests; 90 total.
