Add verify to CheckArgs

This is useful for verifying a return result meets a particular condition
without having to make a temporary variable, check that, and then return the
variable.
rename MapcodeCodecTest to DecoderTest

(since that's what it's really testing, just going through the MapcodeCodec
wrapper.)
Fix test gaps in Encoder/MapcodeCodec

A few things going on in this commit, all part of the quest for 100% coverage.

First of all, we were double-checking arguments in MapcodeCodec; this is
pointless and leads to uncoverable code in Encoder. Now all argument checks are
done in Encoder.

We still need to check responses on the way out of MapcodeCodec since it calls
the Encoder API in various ways, and needs to verify the results in a way
Encoder cannot.
Simplify decodeUTF16 & Unicode2Ascii
Decoder cleanup; test for decodeUTF16
Simpler decode_chars usage
More Decoder cleanup
Remove unreachable code and the log4j.xml test resource
Decoder optimizations