Optimize Marshal for string values (#311)

Performance:

	name                                        old time/op    new time/op    delta
	Testdata/CanadaGeometry/Marshal/Concrete    1.38ms ± 1%    1.39ms ± 1%     ~     (p=0.052 n=10+10)
	Testdata/CitmCatalog/Marshal/Concrete       1.45ms ± 0%    1.46ms ± 1%   +0.82%  (p=0.013 n=9+10)
	Testdata/GolangSource/Marshal/Concrete      5.61ms ± 3%    5.48ms ± 1%   -2.38%  (p=0.001 n=10+10)
	Testdata/StringEscaped/Marshal/Concrete     31.1µs ± 5%    26.8µs ± 1%  -13.84%  (p=0.000 n=10+10)
	Testdata/StringUnicode/Marshal/Concrete     30.9µs ± 3%    26.7µs ± 0%  -13.62%  (p=0.000 n=10+10)
	Testdata/SyntheaFhir/Marshal/Concrete       8.77ms ± 1%    8.50ms ± 3%   -3.11%  (p=0.000 n=10+10)
	Testdata/TwitterStatus/Marshal/Concrete      965µs ± 4%     926µs ± 2%   -4.01%  (p=0.002 n=10+10)

arshal_default.go

@@ -167,7 +167,24 @@ func makeStringArshaler(t reflect.Type) *arshaler {
 		if mo.Format != "" && mo.FormatDepth == xe.Tokens.Depth() {
 			return newInvalidFormatError("marshal", t, mo.Format)
 		}
-		return enc.WriteToken(jsontext.String(va.String()))
+
+		// Optimize for marshaling without preceding whitespace or string escaping.
+		s := va.String()
+		if optimizeCommon && !xe.Flags.Get(jsonflags.Expand) && !xe.Tokens.Last.NeedObjectName() && !jsonwire.NeedEscape(s, xe.EscapeRunes) {
+			b := xe.Buf
+			b = xe.Tokens.MayAppendDelim(b, '"')
+			b = append(b, '"')
+			b = append(b, s...)
+			b = append(b, '"')
+			xe.Buf = b
+			xe.Tokens.Last.Increment()
+			if xe.NeedFlush() {
+				return xe.Flush()
+			}
+			return nil
+		}
+
+		return enc.WriteToken(jsontext.String(s))
 	}
 	fncs.unmarshal = func(dec *jsontext.Decoder, va addressableValue, uo *jsonopts.Struct) error {
 		xd := export.Decoder(dec)