lib/fs: Optimize Canonicalize

When pathSep is a constant, the compiler precomputes pathSep+pathSep and
".."+pathSep instead of emitting function calls to compute "//" and
"../". Benchmark results in lib/osutil:

name                old time/op    new time/op    delta
TraversesSymlink-8    8.86µs ± 3%    8.53µs ± 4%  -3.79%  (p=0.000 n=18+20)

name                old alloc/op   new alloc/op   delta
TraversesSymlink-8    1.06kB ± 0%    1.06kB ± 0%    ~     (all equal)

name                old allocs/op  new allocs/op  delta
TraversesSymlink-8      15.0 ± 0%      15.0 ± 0%    ~     (all equal)

lib/fs/filesystem.go

@@ -253,7 +253,7 @@ func IsInternal(file string) bool {
 // - /foo/bar -> foo/bar
 // - / -> "."
 func Canonicalize(file string) (string, error) {
-	pathSep := string(PathSeparator)
+	const pathSep = string(PathSeparator)
 
 	if strings.HasPrefix(file, pathSep+pathSep) {
 		// The relative path may pretend to be an absolute path within