Implement a weighted list
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.
- Unit tests provided
- The next commit will implement this for AcceptedLanguages
Reviewed By: dereckson
Differential Revision: https://devcentral.nasqueron.org/D1633