X xfmt/qpy: try to reorganizefor common case = ascii printable characters

name       old time/op    new time/op    delta
PyQuote-4     950ns ± 0%     894ns ± 0%  -5.89%  (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

@@ -47,28 +47,31 @@ func AppendQuotePyBytes(buf, b []byte) []byte {
 		switch {
 		// fast path - ASCII only - trying to avoid UTF-8 decoding
 		case c < utf8.RuneSelf:
-			switch c {
-				case '\\', quote:
+			switch {
+				case c == '\\' || c == quote:
 					buf = append(buf, '\\', c)
 
+				case ' ' <= c && c <= '\x7e':
+					// printable ASCII
+					buf = append(buf, c)
+
+
+				// below: non-printable ASCII
+
 				// 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
-				case '\t':
+				case c == '\t':
 					buf = append(buf, `\t`...)
-				case '\n':
+				case c == '\n':
 					buf = append(buf, `\n`...)
-				case '\r':
+				case c == '\r':
 					buf = append(buf, `\r`...)
 
 				default:
-					if c < ' ' || c == '\x7f' /* the only non-printable ASCII character > space */  {
+					// NOTE c < ' ' or c == '\x7f' (the only non-printable ASCII character > space) here
 					// we already converted to \<letter> what python represents as such above
 					// everything else goes in numeric byte escapes
 					buf = append(buf, '\\', 'x', hexdigits[c>>4], hexdigits[c&0xf])
-					} else {
-						// printable ASCII
-						buf = append(buf, c)
-					}
 			}
 
 			b = b[1:]
@@ -77,12 +80,11 @@ func AppendQuotePyBytes(buf, b []byte) []byte {
 		default:
 			r, size := utf8.DecodeRune(b)
 
-			switch r {
-			case utf8.RuneError:
+			switch {
+			case r == utf8.RuneError:
+				// decode error - just emit raw byte as escaped
 				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]...)
@@ -93,7 +95,6 @@ func AppendQuotePyBytes(buf, b []byte) []byte {
 					buf = append(buf, '\\', 'x', hexdigits[b[i]>>4], hexdigits[b[i]&0xf])
 				}
 			}
-			}
 
 			b = b[size:]
 		}