optimize makeFuture and Future<T>::Future()

optimize makeFuture and Future<T>::Future()

Summary: No reason to go through the whole Promise rigamarole. Add an appropriate Core ctor and use that to make a completed Future with just the core alloc

Note the big win in the `constantFuture` benchmark.

```
Before:
============================================================================
folly/futures/test/Benchmark.cpp                relative  time/iter  iters/s
============================================================================
constantFuture                                             120.50ns    8.30M
promiseAndFuture                                  91.99%   130.98ns    7.63M
withThen                                          28.17%   427.77ns    2.34M
----------------------------------------------------------------------------
oneThen                                                    430.48ns    2.32M
twoThens                                          58.03%   741.86ns    1.35M
fourThens                                         31.85%     1.35us  739.97K
hundredThens                                       1.61%    26.80us   37.32K
----------------------------------------------------------------------------
no_contention                                                4.58ms   218.48
contention                                        83.70%     5.47ms   182.86
----------------------------------------------------------------------------
throwAndCatch                                                8.09us  123.55K
throwAndCatchWrapped                              94.43%     8.57us  116.67K
throwWrappedAndCatch                             154.69%     5.23us  191.12K
throwWrappedAndCatchWrapped                      614.06%     1.32us  758.70K
----------------------------------------------------------------------------
throwAndCatchContended                                     967.54ms     1.03
throwAndCatchWrappedContended                    103.48%   935.04ms     1.07
throwWrappedAndCatchContended                    148.24%   652.70ms     1.53
throwWrappedAndCatchWrappedContended            14313.28%     6.76ms   147.94
============================================================================

After:
============================================================================
folly/futures/test/Benchmark.cpp                relative  time/iter  iters/s
============================================================================
constantFuture                                              69.11ns   14.47M
promiseAndFuture                                  55.12%   125.37ns    7.98M
withThen                                          16.49%   419.18ns    2.39M
----------------------------------------------------------------------------
oneThen                                                    370.39ns    2.70M
twoThens                                          55.11%   672.05ns    1.49M
fourThens                                         29.00%     1.28us  782.89K
hundredThens                                       1.23%    30.22us   33.09K
----------------------------------------------------------------------------
no_contention                                                4.56ms   219.46
contention                                        82.82%     5.50ms   181.77
----------------------------------------------------------------------------
throwAndCatch                                                8.30us  120.42K
throwAndCatchWrapped                              96.40%     8.61us  116.08K
throwWrappedAndCatch                             162.66%     5.11us  195.89K
throwWrappedAndCatchWrapped                      680.39%     1.22us  819.36K
----------------------------------------------------------------------------
throwAndCatchContended                                     979.17ms     1.02
throwAndCatchWrappedContended                    103.09%   949.84ms     1.05
throwWrappedAndCatchContended                    153.55%   637.71ms     1.57
throwWrappedAndCatchWrappedContended            10468.47%     9.35ms   106.91
============================================================================
```

Reviewed By: @fugalh, @​hannesr

Differential Revision: D2144664