Speed up walkDir significantly

We only know that this works on Linux and Mac OS X, so other systems use
the POSIX conforming version still. This removed the lstat call, which
is especially expensive on NFS filesystems for me.

lib/posix/posix.nim

@@ -70,6 +70,16 @@ const
   STDIN_FILENO* = 0  ## File number of stdin;
   STDOUT_FILENO* = 1 ## File number of stdout;
 
+  DT_UNKNOWN* = 0
+  DT_FIFO* = 1
+  DT_CHR* = 2
+  DT_DIR* = 4
+  DT_BLK* = 6
+  DT_REG* = 8
+  DT_LNK* = 10
+  DT_SOCK* = 12
+  DT_WHT* = 14
+
 type
   TDIR* {.importc: "DIR", header: "<dirent.h>",
           incompleteStruct.} = object
@@ -84,6 +94,9 @@ type
   Tdirent* {.importc: "struct dirent",
              header: "<dirent.h>", final, pure.} = object ## dirent_t struct
     d_ino*: Tino  ## File serial number.
+    d_off*: TOff
+    d_reclen*: cshort
+    d_type*: int8
     d_name*: array [0..255, char] ## Name of entry.
 
   Tflock* {.importc: "struct flock", final, pure,

lib/pure/os.nim

@@ -1299,10 +1299,14 @@ iterator walkDir*(dir: string): tuple[kind: PathComponent, path: string] {.
         if y != "." and y != "..":
           var s: TStat
           y = dir / y
-          if lstat(y, s) < 0'i32: break
           var k = pcFile
-          if S_ISDIR(s.st_mode): k = pcDir
-          if S_ISLNK(s.st_mode): k = succ(k)
+          when defined(linux) or defined(macosx):
+            if x.d_type == DT_DIR: k = pcDir
+            if x.d_type == DT_LNK: k = succ(k)
+          else:
+            if lstat(y, s) < 0'i32: break
+            if S_ISDIR(s.st_mode): k = pcDir
+            if S_ISLNK(s.st_mode): k = succ(k)
           yield (k, y)
       discard closedir(d)