Add basic futures to observe dispatch of actions and effects

[arximboldi]

This PR addresses #89 and potentially #96.

[codecov-commenter]

Codecov Report

Merging #128 (719e744) into master (5df6dab) will increase coverage by 0.10%.
The diff coverage is 98.42%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   94.94%   95.04%   +0.10%     
==========================================
  Files          73       76       +3     
  Lines        2649     2747      +98     
==========================================
+ Hits         2515     2611      +96     
- Misses        134      136       +2     

Impacted Files	Coverage Δ
lager/debug/debugger.hpp	47.61% <ø> (+0.84%)	⬆️
lager/future.hpp	88.88% <88.88%> (ø)
lager/context.hpp	89.28% <100.00%> (-4.47%)	⬇️
lager/effect.hpp	100.00% <100.00%> (ø)
lager/store.hpp	98.61% <100.00%> (+0.39%)	⬆️
test/core.cpp	98.97% <100.00%> (+0.01%)	⬆️
test/futures.cpp	100.00% <100.00%> (ø)
test/sensor.cpp	100.00% <100.00%> (ø)
test/setter.cpp	100.00% <0.00%> (ø)
... and 3 more

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[arximboldi]

Merging this now.

In a private repo, @isherman has shown that enabling futures can have a performance penalty per-dispatch. The penalty is small enough that it is probably ok for most projects, but it is still worth having it optional. So to enable it, you need to pass the lager::with_futures enhancer to the make_store call. In the future I am gonna explore other designs that either remove the performance penalty or make it optional per dispatch() instead of per store.