Read .mxl as .zip, and get the xml inside, also write .xml with same xml text

.mxl is a compressed version of the .musicxml file.  Despite the extension, it is actually a .zip file.  We should have a musicxml crate in the root of the project.  The first step in creating this crate is to have ZIP file reading and writing.

Read MXL and write MXL zip file.