zstd: Improve best compression's match selection (#705)

The best encoder selects matches based on the criterion

	a.est+(a.s-b.s)*bitsPerByte>>10 < b.est+(b.s-a.s)*bitsPerByte>>10

If this were computed on the reals, it would be equivalent to
a.est < b.est, so the added terms only capture round-off error
(this is also why CSE doesn't eliminate them).

Changing the formula to

	a.est-b.est+(a.s-b.s)*bitsPerByte>>10 < 0

captures the intention better, I think, and improves compression:

	enwik9           260989017 259699309 -0.4942%
	silesia/dickens    3233958   3222189 -0.3639%
	silesia/mozilla   16980973  16912341 -0.4042%
	silesia/mr         3505223   3505553  0.0094%
	silesia/nci        2313702   2289871 -1.0300%
	silesia/ooffice    2915199   2896410 -0.6445%
	silesia/osdb       3364752   3390871  0.7763%
	silesia/reymont    1658404   1656006 -0.1446%
	silesia/samba      4330660   4326783 -0.0895%
	silesia/sao        5399736   5416932  0.3185%
	silesia/webster    9987784   9966351 -0.2146%
	silesia/xml         542081    538378 -0.6831%
	silesia/x-ray      5756210   5733061 -0.4022%

... as well as throughput:

	name                              old speed      new speed      delta
	Encoder_EncodeAllSimple/best-8    12.1MB/s ± 1%  12.2MB/s ± 1%  +1.17%  (p=0.000 n=18+20)
	Encoder_EncodeAllSimple4K/best-8  10.4MB/s ± 1%  10.5MB/s ± 1%  +0.82%  (p=0.000 n=20+20)

zstd/enc_best.go

@@ -190,7 +190,7 @@ encodeLoop:
 		}
 
 		bestOf := func(a, b match) match {
-			if a.est+(a.s-b.s)*bitsPerByte>>10 < b.est+(b.s-a.s)*bitsPerByte>>10 {
+			if a.est-b.est+(a.s-b.s)*bitsPerByte>>10 < 0 {
 				return a
 			}
 			return b