v2.18.0

Validate an import one row at a time, instead of all or nothing.

New

• Excel.validateRows(sheetName, schema) yields a SheetRowValidation per row
  as the sheet is read: the row keyed by header, its sheet row number, and
  everything the schema objected to. A bad row no longer costs the good ones,
  which is what an importer actually needs.
• Built on streamRows, so it is lazy and memory-bounded. Rejecting a file on
  the first bad row means simply stopping the loop; the rest is never read.
• SheetRowValidation.displayRow is the 1-based number a spreadsheet shows, so
  the message a user gets points at the line they can see.
• Validation uses CsvSchema, the same vocabulary the CSV import already uses,
  so a user gets the same verdict whichever format they uploaded.
  CsvValidationException is now re-exported alongside it.