Monoids in C++ - lambda style

After the becpp presentation, some fixes were suggested.

Also, made the monoid a lot nicer to use, with the help of C++ lambda functions

Presented at becpp

As presented at BECPP meeting on Februari 4th, 2019.

Monoids in C++ presentation for becpp

A story of using a design pattern from Function Programming, as always, reducing code through common abstractions.