core/vm: use stack arena

[holiman]

OBS, wip, this is nowhere near ready for merging

This is an experimental stack layout, using an underlying arena of uint256.Int. The actual stacks that are used during execution uses the stack-arena for storage.

At this point, I'm still experimenting with who shoud hold what, e.g. if both the arena and stack-instances need to keep track of top. Right now the arena has a top, and the stacks have a bottom and a size.

I have also not implemented the growing of the arena. Possibly, there will be an ensureCapacity method which ensures that there's room for at least 1 more item (any evm operation grows the stack at most by 1) - and the methods that expand the stack by 1 are the only ones that need ever invoke it.

Since the elements are uint256.Int, we should avoid giving out pointers to the elements. The peek method does so, which is "unsafe", but the peek was only ever meant for evm operations to modify a stack item directly, without popping/pushing. The peeked references are not held more than a very brief time, so it should be ok.

Posting it here for eyes / suggestions / discussions.