- Generally, keep global state to a minimum and add it to
  hilti::rt::detail::GlobalState instead of creating actual globals.
  This applies to static class members, too.

- If you really have to create a global or static class member (which
  should be rare), do not use types that have global constructors. JIT
  is easier if we don't need to support them (and we don't).

- Make sure accesses to state in hilti::rt::detail::GlobalState is
  thread-safe.

  TODO: Actually right now it is not, need to go througn and add
  synchronization where necessary. (Or maybe an accessor object that
  locks the state automatically)

- TODO: The separation of internal details and public API isn't great
  yet. Should move factor out much of the headers into a separate
  `detail/` tree, and then also move more from the public namespace
  into the `detail` namespace.