[lib] Introduce basic permissionsToBitmask and hasPermission

Summary:
These utilities will be used to encode ThreadPermissions as minimally as possible.

Just starting with some utility functions and unit tests.

Test Plan: Naive unit tests pass, will add more.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat, tomek

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9549