Added toString to Appender class
Improve implementation.

Add unittest.
Appender.toString shouldn't pretend it's an array.

So it should still print "Appender!(...)(...)" except that it will now
print the array contents instead of a random memory address.
More concise way to achieve the same thing.
Merge pull request #2425 from quickfur/appender_tostring

Add toString to Appender struct