Allow UnionAll in Unions

[JonasIsensee]

This PR modifies the way Unions are written to file and thus allows saving unions that contain UnionAlls
such as Vector e.g. any parametric struct without the parameterization.

This needs tests!

Important
This is a breaking change.

• Needs a major version number change
• Ideally allow for a compat mode. (Change version in file header and then use old machinery to read if old file header is detected ?)

closes #187
closes #109
closes #65

EDIT
This is now backwards compatible in that old files can still be read!

[JonasIsensee]

@DilumAluthge ,
This particular bit of code has not been touched in years but it seems that you at least had a look at the DataType reconstruction recently.

Would you be willing to inspect my changes?

[DilumAluthge]

Honestly, I don't think I'm qualified to review this PR. It's been a while since I looked at this part of the code, and even then I wasn't very familiar with it.

I looked over the changes and nothing seemed glaringly incorrect, but take that with a huge grain of salt.

[DilumAluthge]

It looks like this change might be backwards-compatible.

That being said, it is a significant change to existing code. Just to be 100% safe, it might be a good idea to make this PR a breaking change, i.e. bump the version number to 0.2.0.

[JonasIsensee]

Thank you for looking at it.
I fear that I may not find people "qualified" and willing to review PRs for most of the things I want to address in the near future.
With more testing I hope I should be able to get things working anyway.

I definitely agree that this not a patch release since files written with this code will not be readable with older JLD2 versions.

[codecov]

Codecov Report

Merging #206 into master will increase coverage by 0.16%.
The diff coverage is 97.36%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   88.86%   89.02%   +0.16%     
==========================================
  Files          19       20       +1     
  Lines        2182     2206      +24     
==========================================
+ Hits         1939     1964      +25     
+ Misses        243      242       -1     

Impacted Files	Coverage Δ
src/JLD2.jl	95.55% <ø> (ø)
src/data.jl	81.45% <97.05%> (+0.41%)	⬆️
src/backwards_compatibility.jl	100.00% <100.00%> (ø)
src/file_header.jl	76.92% <0.00%> (+15.38%)	⬆️

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