Allow to map Vector elements into HashMap

Summary:
Callback must return an array of exactly two elements:
[key, value]

It can takes in parameter ($value) or ($key, $value).

This array is used to build a HashMap with at most the
same number of elements than the vector.

If several identical keys are returned by the different
callback executions, the last value is kept.

This is the reverse operation from D3214.

Test Plan: Unit tests are provided.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3217