ArcString

This is a simple library that contains an atomically reference-counted string type that is only as big as one pointer, but is capable of SSO up to its size. So, when pointers are 64-bit, ArcString is 8 bytes large, and can store strings up to 8 bytes without a heap allocation, and when pointers are 32-bit, that is 4 bytes. It uses NonZeroUsize internally, meaning Option is also only as big as one pointer.