Replace ext-s with git submodules

[cnd]

It will help to update them easier and it's just right thing to do in my opinion.

[leto]

Awesome! +1. Can you update any necessary documentation that needs to be updated with this change, @Heather ?

[tadzik]

This looks great. I'll give it a try later today, and as soon as I verify this works I'll merge it. Thanks!

[tadzik]

How do I update submodules when I just checked out your branch? The ext is empty then, maybe it's also worth a note in the readme

[cnd]

git submodule init
git submodule update

[cnd]

or you can get repo with all submodules by git clone --recursive <reponame>

[cnd]

some projects (for example mono) is doing init & update on configure so I guess it will be fine to make bootstrap to call do it.

[cnd]

I used to note it in readme: https://github.com/Heather/panda/commit/89951258461e86a7d0846e952e521a43bf8c0d58

[tadzik]

Thank you!