Implement rows_* functions

[markfairbanks]

• rows_insert() adds new rows (like INSERT). By default, key values in y must not exist in x.
• rows_append() works like rows_insert() but ignores keys.
• rows_update() modifies existing rows (like UPDATE). Key values in y must be unique, and, by default, key values in y must exist in x.
• rows_patch() works like rows_update() but only overwrites NA values.
• rows_upsert() inserts or updates depending on whether or not the key value in y already exists in x. Key values in y must be unique.
• rows_delete() deletes rows (like DELETE). By default, key values in y must exist in x.