Modify MEMFS to update timestamp of parent directory on file creation and removal.
More POSIXish compliant for a one timestamp fs, behaves like a modification date.

--- a/emsdk/fastcomp/emscripten/src/library_memfs.js    2019-08-19 18:11:40.000000000 +0200
+++ b/emsdk/fastcomp/emscripten/src/library_memfs.js	2020-12-23 12:22:38.000000000 +0100
@@ -88,6 +88,7 @@
       // add the new node to the parent
       if (parent) {
         parent.contents[name] = node;
+        parent.timestamp = node.timestamp;
       }
       return node;
     },
@@ -213,12 +214,16 @@
         }
         // do the internal rewiring
         delete old_node.parent.contents[old_node.name];
+        var now = Date.now();
+        old_node.parent.timestamp = now;
         old_node.name = new_name;
         new_dir.contents[new_name] = old_node;
+        new_dir.timestamp = now;
         old_node.parent = new_dir;
       },
       unlink: function(parent, name) {
         delete parent.contents[name];
+        parent.timestamp = Date.now();
       },
       rmdir: function(parent, name) {
         var node = FS.lookupNode(parent, name);
@@ -226,6 +231,7 @@
           throw new FS.ErrnoError({{{ cDefine('ENOTEMPTY') }}});
         }
         delete parent.contents[name];
+        parent.timestamp = Date.now();
       },
       readdir: function(node) {
         var entries = ['.', '..'];