output: Make table_item a pivot_table, table_cell a pivot_value.

output: Make table_item a pivot_table, table_cell a pivot_value.

This greatly simplifies a lot of glue code and it brings the
internal representation of tables in output closer to the .spv
format representation, which will make better compatibility
possible in the future.

This commit gets rid of render-test, which was very specific
to the lower-level "table" representation, and replaces it
by pivot-table-test, which works in terms of the higher-level
pivot table representation.