builtin: x.vstring() instead of string(x) (#6102)

vlib/os/os.v

@@ -39,7 +39,7 @@ pub fn read_file(path string) ?string {
 		mut str := malloc(fsize + 1)
 		C.fread(str, fsize, 1, fp)
 		str[fsize] = 0
-		return string(str,fsize)
+		return str.vstring_with_len(fsize)
 	}
 }
 
@@ -673,7 +673,7 @@ pub fn get_raw_line() string {
 				}
 				offset++
 			}
-			return string(buf, offset)
+			return buf.vstring_with_len(offset)
 		}
 	} $else {
 		max := size_t(0)
@@ -858,7 +858,7 @@ pub fn executable() string {
 			eprintln('os.executable() failed at reading /proc/self/exe to get exe path')
 			return executable_fallback()
 		}
-		return string(result)
+		return unsafe { result.vstring() }
 	}
 	$if windows {
 		max := 512
@@ -896,7 +896,7 @@ pub fn executable() string {
 			eprintln('os.executable() failed at calling proc_pidpath with pid: $pid . proc_pidpath returned $ret ')
 			return executable_fallback()
 		}
-		return string(result)
+		return unsafe { result.vstring() }
 	}
 	$if freebsd {
 		mut result := vcalloc(max_path_len)
@@ -905,7 +905,7 @@ pub fn executable() string {
 		unsafe {
 			C.sysctl(mib.data, 4, result, &size, 0, 0)
 		}
-		return string(result)
+		return unsafe { result.vstring() }
 	}
 	// "Sadly there is no way to get the full path of the executed file in OpenBSD."
 	$if openbsd {}
@@ -918,7 +918,7 @@ pub fn executable() string {
 			eprintln('os.executable() failed at reading /proc/curproc/exe to get exe path')
 			return executable_fallback()
 		}
-		return string(result,count)
+		return result.vstring_with_len(count)
 	}
 	$if dragonfly {
 		mut result := vcalloc(max_path_len)
@@ -927,7 +927,7 @@ pub fn executable() string {
 			eprintln('os.executable() failed at reading /proc/curproc/file to get exe path')
 			return executable_fallback()
 		}
-		return string(result,count)
+		return unsafe { result.vstring_with_len(count) }
 	}
 	return executable_fallback()
 }
@@ -1056,7 +1056,7 @@ pub fn getwd() string {
 		if C.getcwd(charptr(buf), 512) == 0 {
 			return ''
 		}
-		return string(buf)
+		return unsafe { buf.vstring() }
 	}
 }
 
@@ -1079,7 +1079,7 @@ pub fn real_path(fpath string) string {
 			return fpath
 		}
 	}
-	return string(fullpath)
+	return unsafe { fullpath.vstring() }
 }
 
 // is_abs_path returns true if `path` is absolute.