folly::gen, or Comprehensions->Folly

folly::gen, or Comprehensions->Folly

Summary:
Moving Comprehensions library into Folly and refactoring its interface to be much more modular and composable. There are now two core abstractions:

# Generators: Standalone classes supporting ##apply()## and optionally ##foreach()##. These all inherit from ##GenImpl<T, Self>##.
# Operators: Standalone classes which, when composed with a generator, produce a new generator. These all inherit from ##OperatorImpl<Self>##.

These generators may be composed with ##operator|## overloads which only match ##const GenImpl<T, Self>&## on the left like ##gen | op##.  Additionally, generator may be consumed inline with ##gen | lambda## like ##gen | [](int x) { cout << x << endl; };##.

With this design, new operators may be added very simply without modifying the core library and templates are instantiated only exactly as needed.

Example:

```lang=cpp
auto sum = seq(1, 10) | filter(isPrime) | sum;
seq(1, 10) | [](int i) {
cout << i << endl;
};
```

Test Plan: Unit tests

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D542215