Optimize Encoder.WriteValue for object names (#68)

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