Rename internal Core variants

[frankmcsherry]

We have several "largely internal" types that are suffixed with Core in an abundance of caution about breaking external uses. That caution is now gone, and we are going to start looking at changes that introduce the opinion that containers are the "right" least common denominator approach to using TD, and Vec containers are mostly an ergonomic layer on top of them (and some amount of performance anti-pattern).

There are other Core types that are less internal, things like StreamCore, HandleCore, and others. We'll eventually want to rationalize those as well, but they are likely to be very breaking rather than lightly breaking, and the migration there will probably look more like the type stuff we have at the moment (though likely in some dataflow::vec module, rather than implicitly recommended for everyone).