Move srg writing and symbol comparison into JarMapping
Fix missing main class
Option parsing with joptsimple

Allow optionally writing srg to a file, or to stdout if unspecified.
Add -q option to suppress verbose output
Move .srg writing into SrgWriter
Add 'compact srg' output format

This is a simpler mapping format, somewhat easier to parse, and without
the redundant information in the original .srg file format.