Add source logging to read forms.

- Introduce new SourceLoggingPushbackReader type to reader types.
- All IMeta read by a SourceLoggingPushbackReader have a :source key in their metadata.
- The :source key is the character string which was read to produce the IMeta.
- Tests to verify the :source keys for various kinds of values, e.g. IMetas, nested IMetas, native Java types, IMetas with Meta present.

Also-by: Bronsa <brobronsa@gmail.com>

Signed-off-by: Bronsa <brobronsa@gmail.com>