Adding collection literals to PFFF ast

Summary:
^

`VectorLit` and `MapLit` each accept half of the existing
`array_pair` formats, so implemented accordingly.

Test Plan:
make test

  $ cat ./test.php

    <?hh
    // Copyright 2004-present Facebook. All Rights Reserved.

    <<Injectable>>
    final class ResolvedEntReferenceStorageAdapterValidator {

       public function foo() {
         $x = StableMap { "5" => 5 };
       }
    }

  $ ./pfff -dump_php ./test.php

  [ClassDef(
   {c_type=ClassFinal(i_1, i_2);
    c_name=Name(("ResolvedEntReferenceStorageAdapterValidator", i_3));
    c_extends=None; c_implements=None;
    c_attrs=Some((i_4, [Left(Attribute(("Injectable", i_5)))], i_6));
    c_body=(i_7,
            [Method(
               {f_tok=i_8; f_type=MethodRegular; f_attrs=None;
                f_modifiers=[(Public, i_9)]; f_ref=None;
                f_name=Name(("foo", i_10)); f_params=(i_11, [], i_12);
                f_return_type=None;
                f_body=(i_13,
                        [ExprStmt(
                           Assign(Var(DName(("x", i_14)), Ref(NoScope)),
                             i_15,
                             MapLit(i_16,
                               (i_17,
                                [Left(
                                   MapArrowExpr(Sc(C(String(("5", i_18)))),
                                     i_19, Sc(C(Int(("5", i_20))))))],
                                i_21))), i_22)],
                        i_23);
                })],
            i_24);
    }); FinalDef(i_25)]

Reviewers: pad, andrewparoski

Reviewed By: pad

CC: ps, platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D700450

Task ID: 2057994