Support optionally discarding fields from CSV data rather than including them
in the records returned by the CSV reader.

src/csv.m:
src/csv.csv.typed_reader.m:
	Allow fields to be discarded rather than returned
	as part of a record.

tests/test_cases.m:
tests/discard.{inp,exp}:
tests/discard-bad.{inp,err_exp}:
	Test the new functionality.

src/Makefile:
	Add a tags target.