Reformat Filters

Make parameters final, all under 80 columns etc etc.
Cleanup of org.parboiled.matchervisitors.*
Cleanup of org.parboiled.buffers

In particular, make the documentation _not_ lie: buffers were not immutable!

Now they are (except for MutableInputBuffer, but this one is, uh, special. Is it
useful at all?).
ArrayBuilder: is unused, mark as deprecated, schedule for removal
InstructionGroupPreparer: make inner class MD5Digest immutable
Get rid of the dependency on Base64

And add a comment. We must REALLY get rid of .toString() based tests!
InstructionGroupPreparer, part 1

More to do...
InstructionGroupPreparer, part 2

More to do...

My god this code sucks...
InstructionGroupPreparer, part 3

More to do!
InstructionGroupPreparer, final part for now

But a real tester needs to be written for bytecode tests. String dumps? Meh.