Set a default external enumeration library

[cr-marcstevens]

This PR makes it possible to directly integrate any external enumeration into fplll and be used by default by fplll.

[malb]

Looks good to me, so once the checkstyle issue is fixed, happy to merge.

[cr-marcstevens]

Thanks, but indeed all checks failed, so have to fix style and maybe other things.

[cr-marcstevens]

BTW this pull request moves the extenum API from the fplll namespace to the main namespace ::.
This is because the header file doesn't use any other fplll header files, so that it can be copied to extenum libraries without dependencies.

What are your thoughts on this:

1. Should this be moved back to fplll?
   1a. If yes, do we use macros then or just hardcoded fplll as namespace?

[malb]

TBH, I don't have a strong opinion either way. What's the risk of name collision in some bigger project, i.e. should it be in some namespace?

[codecov]

Codecov Report

Merging #456 (da91584) into master (1ac8c38) will decrease coverage by 0.00%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
- Coverage   64.74%   64.73%   -0.01%     
==========================================
  Files          75       75              
  Lines        7432     7433       +1     
==========================================
  Hits         4812     4812              
- Misses       2620     2621       +1     

Impacted Files	Coverage Δ
fplll/enum-parallel/enumlib.cpp	100.00% <ø> (ø)
fplll/enum/enumerate_base.h	100.00% <ø> (ø)
fplll/enum/enumerate_ext.cpp	77.19% <ø> (ø)
fplll/enum/enumerate_ext.h	44.44% <50.00%> (-5.56%)	⬇️
fplll/enum-parallel/enumlib_dim.cpp	76.92% <66.66%> (ø)
fplll/enum/enumerate.h	79.31% <100.00%> (ø)

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[cr-marcstevens]

@malb do you want to push the green button?

[malb]

With pleasure.