os.dir(); fix vroot on Windows

compiler/main.v

@@ -933,7 +933,7 @@ fn new_v(args[]string) *V {
 	'option.v',
 	]
 	// Location of all vlib files
-	vroot := os.executable().all_before_last('/') 
+	vroot := os.dir(os.executable()) 
 	println('VROOT=$vroot') 
 	// v.exe's parent directory should contain vlib 
 	if os.dir_exists(vroot) && os.dir_exists(vroot + '/vlib/builtin') {

vlib/os/os.v

@@ -382,6 +382,16 @@ pub fn ext(path string) string {
 	return path.right(pos)
 }
 
+
+// dir returns all but the last element of path, typically the path's directory.  
+pub fn dir(path string) string {
+	pos := path.last_index(PathSeparator) 
+	if pos == -1 {
+		return '.' 
+	} 
+	return path.left(pos) 
+} 
+
 fn path_sans_ext(path string) string {
 	pos := path.last_index('.')
 	if pos == -1 {

vlib/os/os_nix.v

@@ -1,3 +1,7 @@
 module os 
 
 #include <dirent.h>
+
+const (
+	PathSeparator = '/' 
+) 

vlib/os/os_test.v

@@ -34,6 +34,15 @@ fn test_write_and_read_string_to_file() {
   os.rm(filename)
 }
 
+fn test_dir() {
+	$if windows {
+		assert os.dir('C:\a\b\c') == 'C:\a\b' 
+ 
+	} $else { 
+		assert os.dir('/var/tmp/foo') == '/var/tmp' 
+	} 
+} 
+
 //fn test_fork() {
 //  pid := os.fork()
 //  if pid == 0 {

vlib/os/os_win.v

@@ -1,5 +1,9 @@
 module os
 
+const (
+	PathSeparator = '/' 
+) 
+
 // Ref - https://docs.microsoft.com/en-us/windows/desktop/winprog/windows-data-types
 // A handle to an object.
 type HANDLE voidptr