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 :^)
Instead of using Strings in the bytecode ops this adds a global string
table to the Executable struct which individual operations can refer
to using indices. This brings bytecode ops one step closer to being
pointer free.