Simplify by creating struct roff_node syntax tree nodes for tbl(7)

Simplify by creating struct roff_node syntax tree nodes for tbl(7)
right from roff_parseln() rather than delegating to read.c,
similar to what i just did for eqn(7).

The interface function roff_span() becomes obsolete and is deleted,
the former interface function roff_addtbl() becomes static,
the interface functions tbl_read() and tbl_cdata() become void,
and minus twelve linus of code.

No functional change.