datatype: Describe rt symbol tables

datatype: Describe rt symbol tables

Implement a symbol_table_print() wrapper for the run-time populated
rt_symbol_tables which formats output similar to expr_describe() and
includes the data source.

Since these tables reside in struct output_ctx there is no implicit
connection between data type and therefore providing callbacks for
relevant datat types which feed the data into said wrapper is a simpler
solution than extending expr_describe() itself.

Signed-off-by: Phil Sutter <phil@nwl.cc>