support for Memory and MemoryRef

[JonasIsensee]

Implements the required special handling of Memory and MemoryRef.

Memory is treated as a regular mutable object and hence, object identity is preserved.
(e.g. when doing

m = Memory{Int}(undef, 10)
mr = memoryref(m, 1)
jldsave("test.jld2"; m, mr)
d = load("test.jld2")
d["m"] === d["mr"].mem

However potential aliasing between Array and Memory objects is ignored.

[codecov]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.36%. Comparing base (4901614) to head (e76af4e).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/dataspaces.jl	33.33%	2 Missing ⚠️
src/datasets.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
+ Coverage   85.26%   85.36%   +0.09%     
==========================================
  Files          37       37              
  Lines        4399     4407       +8     
==========================================
+ Hits         3751     3762      +11     
+ Misses        648      645       -3     

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