Add new vom conversion tests.

While the regular vom tests run against the golden bytes, the
conversion tests use vdltest and perform conversions.  This
exposed a few unimplemented features and bugs; most notably, the
bytes special-case wasn't implemented in xDecoder.

Also updated comments for the new coders.

Change-Id: I2a6326ce8e41425bca2c6c1c44ccc6f801269e58
8 files changed