Summary:
Renaming the `better` utilities to `butter`:
- to prevent claims that this library is superior to others - it really depends on use cases
- to indicate ease of use throughout the codebase, easily spread like butter
Changelog: [C++][Changed] Renaming C++ better util to butter, used by Fabric internals
Reviewed By: JoshuaGross
Differential Revision: D33242764
fbshipit-source-id: 26dc95d9597c61ce8e66708e44ed545e0fc5cff5