encoding/csv: restore Go 1.9 quoted \r\n handling in Reader

CL 52810 changed Reader to interpret a quoted \r\n as a raw \r\n
when reading fields. This seems likely to break existing users, and
discussion on both #21201 (the original issue that triggered the change)
and #22746 (discussing whether to revert the change) failed to identify
a single motivating example for this change. To avoid breaking existing
users for no clear reason, revert the change.

The Reader has been rewritten in the interim so this is not a git revert
but instead and adjustment (and slight simplification) of the new Reader.

Fixes #22746.

Change-Id: Ie857b2f4b1359a207d085b6d3c3a6d440a997d12
Reviewed-on: https://go-review.googlesource.com/78295
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/encoding/csv/reader_test.go

@@ -235,9 +235,9 @@ x,,,
 		Error: &ParseError{StartLine: 2, Line: 5, Column: 0, Err: ErrQuote},
 	}, {
 		Name:  "CRLFInQuotedField", // Issue 21201
-		Input: "\"Hello\r\nHi\"",
+		Input: "A,\"Hello\r\nHi\",B\r\n",
 		Output: [][]string{
-			{"Hello\r\nHi"},
+			{"A", "Hello\nHi", "B"},
 		},
 	}, {
 		Name:   "BinaryBlobField", // Issue 19410