Change the analyzers so that they don't keep the entire file in memory.

Instead of passing a stream to the analyze() methods, and have them
cache the entire contents in a byte or char array, open a new stream
each time the source file should be read.

Also, write the xref files from the analyze() methods. In many
analyzers, this avoids the need for building up a data structure that
holds the full xref output in memory.

This fixes #8.