Base64Converter now uses a connection directly, instead of an analyzer
redirecting to the underlying connection for reporting to Weird. The new
built-in functions en-/decode_base64_intern make use of this to send
encoding-errors to Weird instead of Reporter.
According to the documentation, using the empty string as alphabet in
the built-in functions, will use the default alphabet. Therefore the
built-in functions can now use default arguments and
en-/decode_base64_custom is deprecated.
The tests have been updated accordingly.
