Don't use bytes.Buffer when not necessary (#549)

When parsing strings, they can be referenced directly from the document
when they don't contain escaped characters. This avoids paying to cost
of allocating (and sometimes growing) the bytes buffer unecessarily.

parser.go

@@ -511,8 +511,6 @@ func (p *parser) parseMultilineBasicString(b []byte) ([]byte, []byte, error) {
 		return nil, nil, err
 	}
 
-	var builder bytes.Buffer
-
 	i := 3
 
 	// skip the immediate new line
@@ -522,6 +520,21 @@ func (p *parser) parseMultilineBasicString(b []byte) ([]byte, []byte, error) {
 		i += 2
 	}
 
+	// fast path
+	startIdx := i
+	endIdx := len(token) - len(`"""`)
+	for ; i < endIdx; i++ {
+		if token[i] == '\\' {
+			break
+		}
+	}
+	if i == endIdx {
+		return token[startIdx:endIdx], rest, nil
+	}
+
+	var builder bytes.Buffer
+	builder.Write(token[startIdx:i])
+
 	// The scanner ensures that the token starts and ends with quotes and that
 	// escapes are balanced.
 	for ; i < len(token)-3; i++ {
@@ -673,11 +686,25 @@ func (p *parser) parseBasicString(b []byte) ([]byte, []byte, error) {
 		return nil, nil, err
 	}
 
+	// fast path
+	i := len(`"`)
+	startIdx := i
+	endIdx := len(token) - len(`"`)
+	for ; i < endIdx; i++ {
+		if token[i] == '\\' {
+			break
+		}
+	}
+	if i == endIdx {
+		return token[startIdx:endIdx], rest, nil
+	}
+
 	var builder bytes.Buffer
+	builder.Write(token[startIdx:i])
 
 	// The scanner ensures that the token starts and ends with quotes and that
 	// escapes are balanced.
-	for i := 1; i < len(token)-1; i++ {
+	for ; i < len(token)-1; i++ {
 		c := token[i]
 		if c == '\\' {
 			i++