Convert from submodules to subtrees

[matthiasgoergens]

Subtrees are easier to work with. We can still pull from and push to the upstream repositories via the subtree mechanisms.

FYI, conversion works like this:

git submodule deinit circ_blocks
git rm circ_blocks
git commit -m "Remove circ_blocks"

git subtree add --prefix=circ_blocks/ git@github.com:Jiangkm3/circ_blocks.git master

Same for the other submodules. Be careful to check that the branches are correct. For ff it's main.

We can always convert back later, if we need to.