read compat for many hdf5 files

[JonasIsensee]

This PR implements significant additions to JLD2.
It allows read access to many hdf5 files written with by the standard hdf5 c library.

It requires decoding of the fractal heap, v2 b-btree and various messages in different versions compared to what is used by jld2.

[codecov]

Codecov Report

Base: 88.57% // Head: 84.06% // Decreases project coverage by -4.50% ⚠️

Coverage data is based on head (f017e4f) compared to base (35baacc).
Patch coverage: 76.66% of modified lines in pull request are covered.

❗ Current head f017e4f differs from pull request most recent head 8bdf597. Consider uploading reports for the commit 8bdf597 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
- Coverage   88.57%   84.06%   -4.51%     
==========================================
  Files          29       30       +1     
  Lines        2896     4098    +1202     
==========================================
+ Hits         2565     3445     +880     
- Misses        331      653     +322     

Impacted Files	Coverage Δ
src/backwards_compatibility.jl	37.00% <34.78%> (-18.56%)	⬇️
src/object_headers.jl	38.61% <37.20%> (-42.88%)	⬇️
src/data/reconstructing_datatypes.jl	76.43% <44.44%> (-0.94%)	⬇️
src/misc.jl	76.82% <56.75%> (-16.51%)	⬇️
src/global_heaps.jl	93.90% <57.14%> (-3.51%)	⬇️
src/data/number_types.jl	73.68% <60.00%> (-20.44%)	⬇️
src/inlineunion.jl	93.33% <66.66%> (-0.22%)	⬇️
src/data/specialcased_types.jl	90.20% <76.00%> (-5.83%)	⬇️
src/JLD2.jl	89.65% <78.26%> (-2.90%)	⬇️
src/data/writing_datatypes.jl	96.80% <80.00%> (-0.32%)	⬇️
... and 21 more

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