MPS: hash tables / obarrays

[Gerd Möllmann]

I have pushed a change that allocates key/value vectors for hash tables
from MPS, something that had to be done at some point to gain exclusive
access to these vectors when scanning them.

Same is true for obarrays.

I think obarrays in the dump where not made exact roots before. so this
should work much better than before.

If someone could check the obarray related changes I made, that would be
nice.