PEG spans_removeserial--benja: Remove span serialization

While developing the saving/loading functionality as present in 0.8alpha1, the first thing we tried was to use Java serialization. It was always apparent that using Java serialization has two big disadvantages: that it is a) a Java-specific format, tailored to this specific platform, with no implementations on other platforms; and b) not human-readable. On the other hand, Tuomas hopes that serialization is very efficient compared to other formats.

When the implementation was done, I realized a third gotcha: for serialization to work, classes must remain structurally unchanged. This means that all serialized classes must be FROZEN, because any change to an existing class would break loading of serialized data. This means: NO refactoring of the gzz.media.impl package. You can add new classes, but not change/rename/move existing ones.

However, gzz.media.impl is not in a good shape currently. For example, there is far too much stuff inside ScrollBlockManager, degrading readability. And many classes are badly named.

The benefit of clearer code wasn't realized when using serialization, either: all the serialization special-casing in gzz.media.impl makes the code much worse, and to me (having written it) it is actually much harder to understand than the YAML writer/reader, which does all the encoding/decoding to/from YAML in one central monolithic place.

Therefore, I very much think that using Java serialization, which depends a lot on the particular implementation used, is the wrong way to do things. Currently it is there as an alternative to the default, YAML-based file format. I suggest getting rid of serialization, possibly inventing a Java-independent binary format in the future if the human-readable YAML-based approach turns out to be too slow.

This means removal of Media*Stream and the serializing functionality in ScrollBlockManager's inner classes.