We'll now try to detect crashes that were due to dereferencing nullptr,
uninitialized malloc() memory, or recently free()'d memory.
It's not perfect but I think it's pretty good. :^)
Also added some color to the most important parts of the crash log,
and added some more modes to /bin/crash for exercising this code.
Fixes#243.
It can't be 100% precise but it doesn't really matter. Use this to implement
realloc() nicely. This also fixes a bug in realloc() where we didn't take
the size of the allocation metadata into account when computing the size of
an allocation backed by a BigAllocationBlock.
We were only reusing the existing allocation if the new requested size
was exactly the same as the fudged size of the block. This meant that
realloc() could allocate a new block even though the new block would be
identical to the old block.
At the moment, both mmap() and munmap() are kind of slow. Compiling with GCC
was suffering quite badly from munmap() slowness, so let's keep a few more
of the ChunkedBlocks around after they are empty, to avoid having to munmap.
