Reimplement folly::Function to improve compile times.

Reimplement folly::Function to improve compile times.

Summary:
folly::Function is causing significant compile time regressions. Reimplement it in a simpler way.

These are the times for a file containing 1000 instantiations of folly::Fuction (old), folly::Function (new), and std::function with **g++ 4.8 -O3** on my CentOS7 server.

|        | Old `folly::Function` | `std::function` | New `folly::Function` |
|--------|-----------------------|-----------------|-----------------------|
| Time   | 10m37s                | 0m16.81s        | 0m14.75s              |

And for the executable size:

|        | Old `folly::Function` | `std::function` | New `folly::Function` |
|--------|-----------------------|-----------------|-----------------------|
| Size   | 10,409,504            | 732,150         | 562,781               |

That's a **43X** improvement in compile times and an **18X** reduction in executable bloat over the old implementation.

The times for **clang (trunk)** are very different:

|       | Old `folly::Function` | `std::function` | New `folly::Function` |
|-------|-----------------------|-----------------|-----------------------|
| Time  | 4m6s                  | 0m45.27s        | 0m11.78s              |

That's a **20X** improvement over the old implementation and almost a **4X** improvement over `std::function`.

For **gcc-5.3.0**, compile times are again different:

|       | Old `folly::Function` | `std::function` | New `folly::Function` |
|-------|-----------------------|-----------------|-----------------------|
| Time  | 2m49s                 | 0m18.99s        | 0m20.70s              |

With gcc-5.3, the new implementation "only" compiles 8x faster than the old one, and is roughly the same as `std::function`.

Reviewed By: spacedentist, ot, luciang

Differential Revision: D3199985

fb-gh-sync-id: b97982a9dc3a63140510babea34988932e89f2d9
fbshipit-source-id: b97982a9dc3a63140510babea34988932e89f2d9