Connect our serialization with Julia serialization

[antonydellavecchia]

Minimal working example of distributed processing in oscar.
Needs thofma/Hecke.jl#1219 before merge due to conflicting
serialize methods.

Extends serialize for Oscar types that aren't julia types. Introduces two Oscar serializer types
The JSONSerializer is how the serialization in Oscar has ben used so far. The IPCSerializer
aims at being efficient by not sending the refs, which then leaves it up to the user
to guarantee that the appropriate refs are sent to processes that need them.

See included test for an example.

[antonydellavecchia]

This was merged in Hecke thofma/Hecke.jl#1219
and the tests require a Hecke release to pass

[thofma]

There is already a new version. I have restarted CI.

[fingolfin]

It seemed OK on a quick look, let's merge and refine later

[codecov]

Codecov Report

Merging #2837 (587bd67) into master (081c60c) will increase coverage by 0.06%.
Report is 30 commits behind head on master.
The diff coverage is 98.43%.

@@            Coverage Diff             @@
##           master    #2837      +/-   ##
==========================================
+ Coverage   80.62%   80.68%   +0.06%     
==========================================
  Files         456      456              
  Lines       64703    69619    +4916     
==========================================
+ Hits        52166    56175    +4009     
- Misses      12537    13444     +907     

Files	Coverage Δ
src/Serialization/serializers.jl	100.00% <100.00%> (ø)
src/Serialization/main.jl	87.81% <98.07%> (+0.38%)	⬆️

... and 55 files with indirect coverage changes