bytes, internal/bytealg: simplify Equal

The compiler has advanced enough that it is cheaper
to convert to strings than to go through the assembly
trampolines to call runtime.memequal.

Simplify Equal accordingly, and cull dead code from bytealg.

While we're here, simplify Equal's documentation.

Fixes #31587

Change-Id: Ie721d33f9a6cbd86b1d873398b20e7882c2c63e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/173323
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bytes/bytes.go

@@ -12,23 +12,12 @@ import (
 	"unicode/utf8"
 )
 
-// Equal returns a boolean reporting whether a and b
+// Equal reports whether a and b
 // are the same length and contain the same bytes.
 // A nil argument is equivalent to an empty slice.
 func Equal(a, b []byte) bool {
-	return bytealg.Equal(a, b)
-}
-
-func equalPortable(a, b []byte) bool {
-	if len(a) != len(b) {
-		return false
-	}
-	for i, c := range a {
-		if c != b[i] {
-			return false
-		}
-	}
-	return true
+	// Neither cmd/compile nor gccgo allocates for these string conversions.
+	return string(a) == string(b)
 }
 
 // Compare returns an integer comparing two byte slices lexicographically.