[perl #47011] [DEPRECATED] VTABLE entry 'new_from_string'

[Allison Randal via RT]

Fix committed in r35576. It now iterates over an actual C string, so
it's not depending on the internal guts of a STRING being null-terminated.

I can't test the change under --gc=libc because Parrot won't compile (on
Mac or Linux) when configured with --gc=libc. (Which may end up being
deprecated anyway, but the mini code-cleanup here was still a good one
to make.)

Once the 'new_from_string' vtable function has been renamed to something
sane, this ticket can be closed.

Allison