Replace Circular with Oval and add Miri tests

[Posnet]

Oval is a nicer, maintained fork of Circular, that removes the unsafe and unsound code (it replaces them with stdlib features that didn't exist at the time of Circular's writing such as copy_within, so it should generate the same code). Replacing Ciruclar with Oval means we can now add Miri tests to check for undefined/unsound behavior in unsafe rust (even though there isn't in this crate).