This will allow some one constructing an abstract to
only make it visible to the garbage collector after it
is in a valid state. If code in the constructing cfunction
panics before janet_abstract_end is called, the GC will not try
to mark the incomplete abstract type. This is often not needed through
careful programming, but should work well.
Before, we used a local setjmp/longjmp for error handling.
Using janet_panic means errors can be more easily expressive and
code can be smaller.
However, we still need to make vector memory get gc collected, as
panics can cause the runtime to skip janet_v_frees.
