fix warning when compiling cmd/tools/vast/vast.v, add description of -s to ast.txt too

Author: spytheman

cmd/tools/vast/vast.v

@@ -79,7 +79,7 @@ fn (ctx Context) skip_empty(child &Node) bool {
 				return true
 			}
 			if slen > 7 {
-				s := child.valuestring.vstring_with_len(slen)
+				s := unsafe { child.valuestring.vstring_with_len(slen) }
 				if s.starts_with('enum:0(') {
 					return true
 				}

vlib/v/help/other/ast.txt

@@ -21,6 +21,7 @@ Options:
                              File, *AND* generate a .c file too on any change
   -t, --terse                terse output, only with tree node names
                              (AST structure), no details
+  -s, --skip-defaults        skip properties that have default values like [], {}, false, 0, ""
   --hide <multiple strings>  Hide the specified fields. You can give several, by
                              separating them with `,`
   -h, --help                 Display this help and exit