Variable-length stack allocated arrays

[dsimcha]

== Quote from Lars T. Kyllingstad (public at kyllingen.NOSPAMnet)'s > > It's got the
following advantages over alloca:
> >
> > 1.  Can't overflow unless you're completely out of address space.  As a last
> > resort, it allocates another chunk of memory from the heap.
> When all chunks in a block have been TempAlloc.free()'ed, is the block
> GC.free()'ed as well?  Or is the memory retained for future use?  (And
> if the answer to the last question is 'no', is there a reason not to
> retain the memory until the program exits, or at least to include an
> option to?)
> -Lars

The memory is freed according to heuristics.  Here's a brief description:

1.  All state is thread-local.  Unless TempAlloc needs to go to the main heap,
there is never any type of synchronization.

2.  Each chunk is 4 MB.  I've realized that this may be overkill and am thinking
of reducing it to 1 MB or 512K.  This is controlled by an enum, so it would be a
trivial change.

3.  Chunks are stored in a stack (yes, a stack of stacks).  Half of the unused
chunks are freed anytime there are 2x as many unused chunks as used chunks.
Usually 4MB per thread in temp buffer space is plenty, though.