Update Arb to 2.1.0

I am using the git commit taken from the 2.1.0 tag:

https://github.com/fredrik-johansson/arb/releases/tag/2.1.0
Update Flint to 2.4.4

This uses the official tarball. Previously we had to use Fredrik's personal
copy, as that was the only one compatible with Arb. Now the official Flint is
compatible as well, so we should use it from now on.
Merge pull request #324 from hashdist/arb

Update Arb and Flint