Refactor compressing arrays for VTU output.

[bangerth]

First step in a mini project I'm allowing myself today :-) My goal is to generate different parts of VTU files in parallel, rather than writing them sequentially to a stream. As a consequence, I will be moving towards building strings, rather than using streams.

/rebuild

[bangerth]

First step for #14403.

[bangerth]

The cuda tester fails with this message:

The user named '~ginggs' has no PPA named 'ubuntu/deal.ii-9.2.0-backports'

I don't believe that this is related to the PR here.

[marcfehling]

Looks good to me.

Unrelated to this PR: Is there any reason against using Utilities::pack?

[bangerth]

Yes: pack() makes no guarantee how exactly it packs data (or whether it does so at all) except that it promises that you can unpack the data with unpack(). But here, the VTU format specifically prescribes how the compression has to happen, and that the resulting data then has to be uuencoded.