How can I Efficiently Check if the Running User has a Custom Permission?

As of the Winter 18 Release, we can now use the FeatureManagement class for this requirement. The above method can be removed entirely and I can simply call:

Boolean hasPermission = FeatureManagement.checkPermission('CanPerformSomeOperation');

Thanks to @sfdcfox for pointing this one out.

Adrian Larson's answer of using FeatureManagement's static Boolean checkPermission(String customPermissionDeveloperName) will Efficiently Check if the Running User has a Custom Permission.

Testing Apex when using FeatureManagement's static Boolean checkPermission(String customPermissionDeveloperName) isn't so simple. Often, using Custom Permissions will grant a User something private or special, and I want to make sure my Unit Test is doing what I think it's doing when a User both has and doesn't have the Custom Permission(s) in question. So, I made a github repo, TestCustomPermissions, to automate adding Custom Permissions. See README.md for documentation and example.