Implement a weighted list

Summary:
A weighted value is a pair defined by a value, and an associated weight.

A weighted list is a collection of weight values.

This change allows to create a list of items, with an associated weight.

A use case is to parse $_SERVER['HTTP_ACCEPT_LANGUAGE'], a list of
languages, with associated weight to indicate user preferences of
languages with more granularity.

Test Plan:

• Unit tests provided
• The next commit will implement this for AcceptedLanguages

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1633