X xfmt/py: Speedup

it is annoying to see this function high in fstail profiles taking up to 10%

name       old time/op    new time/op    delta
PyQuote-4    1.33µs ± 1%    0.98µs ± 0%  -26.77%  (p=0.008 n=5+5)

name       old alloc/op   new alloc/op   delta
PyQuote-4     0.00B          0.00B          ~     (all equal)

name       old allocs/op  new allocs/op  delta
PyQuote-4      0.00           0.00          ~     (all equal)

t/neo/xcommon/xfmt/python.go

@@ -31,23 +31,20 @@ func AppendQuotePyBytes(buf, b []byte) []byte {
 	// smartquotes: choose ' or " as quoting character
 	// https://github.com/python/cpython/blob/v2.7.13-116-g1aa1803b3d/Objects/stringobject.c#L947
 	quote := byte('\'')
-	noquote := byte('"')
 	if bytes.ContainsRune(b, '\'') && !bytes.ContainsRune(b, '"') {
-		quote, noquote = noquote, quote
+		quote = '"'
 	}
 
 	buf = append(buf, quote)
 
 	for len(b) > 0 {
-		r, size := utf8.DecodeRune(b)
-
-		switch r {
-		case utf8.RuneError:
-			buf = append(buf, '\\', 'x', hexdigits[b[0]>>4], hexdigits[b[0]&0xf])
-		case '\\', rune(quote):
-			buf = append(buf, '\\', byte(r))
-		case rune(noquote):
-			buf = append(buf, noquote)
+		c := b[0]
+		switch {
+		// fast path - ASCII only - trying to avoid UTF-8 decoding
+		case c < utf8.RuneSelf:
+			switch c {
+				case '\\', quote:
+					buf = append(buf, '\\', c)
 
 				// NOTE python converts to \<letter> only \t \n \r  (not e.g. \v)
 				// https://github.com/python/cpython/blob/v2.7.13-116-g1aa1803b3d/Objects/stringobject.c#L963
@@ -59,15 +56,28 @@ func AppendQuotePyBytes(buf, b []byte) []byte {
 					buf = append(buf, `\r`...)
 
 				default:
-			switch {
-			case r < ' ':
+					if c < ' ' || c == '\x7f' /* the only non-printable ASCII character > space */  {
 						// we already converted to \<letter> what python represents as such above
-				buf = append(buf, '\\', 'x', hexdigits[b[0]>>4], hexdigits[b[0]&0xf])
+						// everything else goes in numeric byte escapes
+						buf = append(buf, '\\', 'x', hexdigits[c>>4], hexdigits[c&0xf])
+					} else {
+						// printable ASCII
+						buf = append(buf, c)
+					}
+			}
 
-			case r < utf8.RuneSelf /* RuneSelf itself is not printable */ - 1:
-				// we already escaped all < RuneSelf runes
-				buf = append(buf, byte(r))
+			b = b[1:]
 
+		// slow path - full UTF-8 decoding
+		default:
+			r, size := utf8.DecodeRune(b)
+
+			switch r {
+			case utf8.RuneError:
+				buf = append(buf, '\\', 'x', hexdigits[c>>4], hexdigits[c&0xf])
+
+			default:
+				switch {
 				case strconv.IsPrint(r):
 					// printable utf-8 characters go as is
 					buf = append(buf, b[:size]...)
@@ -82,6 +92,7 @@ func AppendQuotePyBytes(buf, b []byte) []byte {
 
 			b = b[size:]
 		}
+	}
 
 	buf = append(buf, quote)
 	return buf