syntax: use fewer tabwriter escape characters

name     old time/op    new time/op    delta
Print-8    11.5µs ± 0%    11.4µs ± 1%   -1.03%  (p=0.002 n=6+6)

name     old alloc/op   new alloc/op   delta
Print-8      344B ± 0%      312B ± 0%   -9.30%  (p=0.002 n=6+6)

name     old allocs/op  new allocs/op  delta
Print-8      10.0 ± 0%       9.0 ± 0%  -10.00%  (p=0.002 n=6+6)

We'll leave the tabwriter optimizations here for now. I've sent
https://go-review.googlesource.com/c/go/+/166797 to upstream, which
shaves off 27% from our benchmark. In total, the overhead of tabwriter
goes down from the initial ~135% to ~55%, which is much more acceptable.
We even allocate less garbage, too.

name     old time/op    new time/op    delta
Print-8    5.29µs ± 0%    8.21µs ± 1%  +55.19%  (p=0.002 n=6+6)

name     old alloc/op   new alloc/op   delta
Print-8      344B ± 0%      312B ± 0%   -9.30%  (p=0.002 n=6+6)

name     old allocs/op  new allocs/op  delta
Print-8      13.0 ± 0%       9.0 ± 0%  -30.77%  (p=0.002 n=6+6)

syntax/printer.go

@@ -167,7 +167,7 @@ func (c *colCounter) Reset(w io.Writer) {
 type Printer struct {
 	bufWriter
 	tabWriter *tabwriter.Writer
-	cols colCounter
+	cols      colCounter
 
 	indentSpaces   uint
 	binNextLine    bool
@@ -297,16 +297,16 @@ func (p *Printer) writeLit(s string) {
 		p.WriteString(s)
 		return
 	}
+	p.WriteByte('\xff')
 	for i := 0; i < len(s); i++ {
 		b := s[i]
 		if b != '\t' {
 			p.WriteByte(b)
 			continue
 		}
-		p.WriteByte('\xff')
 		p.WriteByte(b)
-		p.WriteByte('\xff')
 	}
+	p.WriteByte('\xff')
 }
 
 func (p *Printer) incLevel() {
@@ -1188,9 +1188,13 @@ func (e *extraIndenter) WriteByte(b byte) error {
 	if b != '\n' {
 		return nil
 	}
-	trimmed := bytes.TrimLeft(e.curLine, "\xff\t")
-	// divided by 3, as each tab is escaped via "\xff\t\xff"
-	lineIndent := (len(e.curLine) - len(trimmed)) / 3
+	line := e.curLine
+	if bytes.HasPrefix(e.curLine, []byte("\xff")) {
+		// beginning a multiline sequence, with the leading escape
+		line = line[1:]
+	}
+	trimmed := bytes.TrimLeft(line, "\t")
+	lineIndent := len(line) - len(trimmed)
 	if e.firstIndent < 0 {
 		e.firstIndent = lineIndent
 		e.firstChange = e.baseIndent - lineIndent