The compiler now always reserves slot zero for either the closure or method
receiver, eliminating a corner case where top-level REPL code lacked the
pre-allocated slot that function/method bodies expect. This ensures local
variables declared in REPL loops (e.g. `for (i in 1..2)`) get correct slot
indexes.
Additionally, inlines `wrenResetFiber` into `wrenNewFiber` and stores the
imported module's closure on the stack before invoking it, preventing a GC
from collecting the closure during import.