control libffi dependency with SOUFFLE_USE_LIBFFI

[quentin]

Hi, the goal of this change is allow building Souffle without libffi and still provide some user-defined functors capabilities in interpreted mode.

When SOUFFLE_USE_LIBFFI=OFF:

• stateful user-defined functors are supported in interpreted mode up to arity 16.
• stateless user-defined functors are supported in interpreted mode up to arity 2 (all combinations of return and argument types available).
• those arity limits are hard-coded but easy to modify.

When SOUFFLE_USE_LIBFFI=ON (default):

• the user-defined functors are called using the non-libffi method when arity allows it.
• otherwise the libffi method is used.

This change also moves some operations to avoid repeating them at each call of a user-defined functor:

• functor libraries are loaded before node generation.
• functor C functions are looked-up during node generation and the function pointer is stored in the node.
• libffi dynamic call structures are prepared in advance during node generation and stored in the node.

[codecov]

Codecov Report

Merging #2188 (f1b1eee) into master (bfc54a5) will decrease coverage by 0.08%.
The diff coverage is 78.78%.

@@            Coverage Diff             @@
##           master    #2188      +/-   ##
==========================================
- Coverage   75.59%   75.51%   -0.09%     
==========================================
  Files         452      452              
  Lines       27259    27355      +96     
==========================================
+ Hits        20607    20657      +50     
- Misses       6652     6698      +46     

Impacted Files	Coverage Δ
src/ast/Aggregator.h	100.00% <ø> (ø)
src/ast/Clause.h	100.00% <ø> (ø)
src/ast/FunctorDeclaration.cpp	38.09% <ø> (ø)
src/ast/analysis/typesystem/Type.cpp	45.84% <0.00%> (ø)
src/ast/analysis/typesystem/TypeSystem.h	84.33% <0.00%> (ø)
src/ast/transform/InlineRelations.cpp	66.10% <0.00%> (ø)
src/ast/transform/RemoveRedundantSums.cpp	50.00% <0.00%> (ø)
src/ast/utility/SipsMetric.cpp	21.33% <ø> (ø)
src/ast/utility/Utils.cpp	91.47% <ø> (ø)
src/ast/utility/Utils.h	100.00% <ø> (ø)
... and 50 more

[b-scholz]

Thanks for the contribution!