Memento: Speed improvements.

Avoid searching the linked list of blocks in order to remove a
block by moving to a doubly linked list. This can be done
without increasing the amount of memory in use by making better
use of the 'parent' pointer that is only used when displaying
nested blocks.

Also store magic values in the 'child' and 'sibling' pointers
(again only used when displaying nested blocks) so that we
can quickly verify that a block is real before doing too much
with it.

Those changes drastically reduce the time required for
MEMENTO_LEAKONLY runs (now the same order of magnitude as non
memento runs).

Normal memento runs are still very slow when the numbers of
blocks increase due to the paranoid checking taking time.

To ameliorate this a bit, we try 2 other things.

Firstly, we optimise the searching of blocks by making use of
int aligned tests. This still doesn't make much difference.

Secondly, we introduce a new mechanism for the 'paranoia'
levels. If a negative number is given for the paranoia level
(say -n) then we first perform our overwrite checks after n events.
We next test after 2n events, then 4n, then 8n etc.

The new default paranoia level is set to be -1024.

This makes a huge difference, and brings normal memento runs
down to be comparable with debug runs.

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