Optimize Encoder.WriteValue for object names

By specifying the isVerbatim flag to names.insertQuoted,
the method can avoid unquoting the name since it knows that
it only needs to strip off the surrounding double quotes.

Performance:

    name                                           old speed      new speed      delta
    Testdata/CanadaGeometry/Encode/Value/Buffered  1.12GB/s ± 0%  1.15GB/s ± 3%     ~     (p=0.151 n=5+5)
    Testdata/CitmCatalog/Encode/Value/Buffered     1.76GB/s ± 0%  1.99GB/s ± 1%  +13.19%  (p=0.008 n=5+5)
    Testdata/GolangSource/Encode/Value/Buffered     793MB/s ± 4%   949MB/s ± 1%  +19.68%  (p=0.008 n=5+5)
    Testdata/StringEscaped/Encode/Value/Buffered    228MB/s ± 1%   229MB/s ± 0%     ~     (p=0.421 n=5+5)
    Testdata/StringUnicode/Encode/Value/Buffered    983MB/s ± 0%  1014MB/s ± 1%   +3.12%  (p=0.008 n=5+5)
    Testdata/SyntheaFhir/Encode/Value/Buffered     1.39GB/s ± 4%  1.65GB/s ± 1%  +18.43%  (p=0.008 n=5+5)
    Testdata/TwitterStatus/Encode/Value/Buffered   1.03GB/s ± 1%  1.23GB/s ± 0%  +19.81%  (p=0.008 n=5+5)

jsontext/encode.go

@@ -716,17 +716,18 @@ func (e *encoderState) reformatObject(dst []byte, src Value, depth int) ([]byte,
 			return dst, n, io.ErrUnexpectedEOF
 		}
 		m := jsonwire.ConsumeSimpleString(src[n:])
-		if m > 0 {
+		isVerbatim := m > 0
+		if isVerbatim {
 			dst = append(dst, src[n:n+m]...)
 		} else {
 			dst, m, err = jsonwire.ReformatString(dst, src[n:], &e.Flags)
 			if err != nil {
 				return dst, n + m, err
 			}
 		}
-		// TODO: Specify whether the name is verbatim or not.
-		if !e.Flags.Get(jsonflags.AllowDuplicateNames) && !names.insertQuoted(src[n:n+m], false) {
-			return dst, n, newDuplicateNameError(src[n : n+m])
+		quotedName := src[n : n+m]
+		if !e.Flags.Get(jsonflags.AllowDuplicateNames) && !names.insertQuoted(quotedName, isVerbatim) {
+			return dst, n, newDuplicateNameError(quotedName)
 		}
 		n += m