Append-only-vec

Note: currently there are frequent CI failures above, which are simply due to failure to install miri to run the test. The tests do pass when run locally.

This crate defines a single data simple structure, which is a vector to which you can only append data. It allows you to push new data values even when there are outstanding references to elements of the AppendOnlyVec. Reading from a AppendOnlyVec is much faster than if it had been protected by a std::sync::RwLock.