Add a "binary integer casing" linter rule

Summary: Adds a linter rule which ensures that binary integers are written in lowercase (i.e. 0b1 instead of 0B1).

Test Plan: Added test cases.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14644