merge rewrite-i2p-i2p into master

[infinity0]

• the master branch has the source history from freenet (both fred and contrib)
• the rewrite-i2p-i2p branch has the source history from i2p. this needs to be merged into the master branch.

in all branches, c code has been moved to src/c/, and history has been rewritten to match this.

i've made a manual merge point (using .git/info/grafts and then git filter-branch -- --all) at merge-point-i2p, which should make 3-way merging slightly easier.

additionally, all binary blobs (contrib-staging had contained them) have been removed. they can be stored in a separate repository.

[infinity0]

BTW, it is possible that the (merge-point-i2p) is not the best place to put the common-ancestor; if someone finds a better one, i can just edit the scripts in git-migrate and re-run it.