Added debug code to dump data_map tree.

- New cpptempl::dump_data() function that takes a data_ptr.
- New dump() methods on cpptempl::Data classes.
- Added internal cpptempl::impl::indent() function to generate an
  indent string of spaces.

Signed-off-by: Chris Reed <flit@me.com>
Fixed empty() pseudo-function.

Signed-off-by: Chris Reed <flit@me.com>
Documentation updates.

Signed-off-by: Chris Reed <flit@me.com>