Add CsvSchema.coerce, CsvTable.coerce, and CsvCodec.decodeWithSchema to
convert each column's values to the type declared on its CsvColumnDef
(int, double, num, bool, String, DateTime). A column with no schema entry
or a null type is left unchanged; CsvTable.coerce returns a copy and never
mutates the source.

Coercion throws CsvParseException (carrying the 0-based row and column) on
a value that cannot convert, or a null in a column declared nullable:false;
a null in a nullable column stays null. Completes the schema story: it
could already validate types, and can now coerce them.