We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Classes reading and writing to the data heap would communicate directly
with the Heap object, and transfer ByteBuffers back and forth with it.
This makes things like caching and locking hard. Therefore all data
persistence activity will be funneled through a Serializer object which
in turn submits it to the Heap.
Introducing this unfortunately resulted in a huge amount of churn, in
which a number of smaller refactorings got caught up as well.
