Possible speed improvement when creating trees

[mmmulani]

Was trying to use webtreemap on a listing that 500,000 files and found that it was hanging in treeify

Was able to speed it up so that it finished in a few seconds, in particular: https://github.com/danvk/webtreemap/blob/master/src/tree.ts#L43-L50 can be changed

from

f (!t.children) t.children = [];
let child = t.children.find(c => c.id === id);
if (!child) {
  child = {id, size: 0};
  t.children.push(child);
}

to

f (!t.children) {
  t.children = [];
  t.childMap = {};
}
let child = t.childMap[id];
if (!child) {
  child = {id, size: 0};
  t.children.push(child);
  t.childMap[id] = child;
}

Of course, it would be good to also clean up the childMap before serialization is called.