Make width calculations occur within tbl_term.c, not tbl.c. This allows

Make width calculations occur within tbl_term.c, not tbl.c.  This allows
for front-ends to make decisions about widths, not the back-end.

To pull this off, first make each tbl_head contain a unique index value
(0 <= index < total tbl_head elements) and remove the tbl_calc() routine
from the back-end.

Then, when encountering the first tbl_span in the front-end, dynamically
create an array of configurations (termp_tbl) keyed on each tbl_head's
unique index value.  Construct the decimals and widths at this time,
then continue parsing as before.

The termp_tbl and indexes are required because we pass a const tbl AST
into the front-end.