Add a linter rule for hexadecimal number casing

Summary: Hexadecimal numbers should be written as 0xFF and not 0xff or 0Xff.

Test Plan: Added test cases.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14612