simple-xml: read xml to hash, write xml from lists.

Racket build-in xml decoding function is: xml->xexpr. But decoded result is a hierrachical list, not covenient use.

Use xml->hash, you can use hash-ref to get xml element directly, or use loop to traverse all the list elements.

Use lists->xml(lists->compact_xml), convert recursive list to generate xml, remove redundant format, more readable.

Chen Xiao <chenxiao770117@gmail.com>