Feedback on new support for more than one look-ahead tokens

[springcomp]

@atifaziz here is a quick PR on top of your multi-peek branch.

My question is what is the purpose of the default case in the Peek() method ?
It seems to me that support for more than two look-ahead tokens is guaranteed to throw an exception.

I added a simple test illustrate this.

I like the ITokenStream abstraction that as been introduced.

As a general question why limit the implementation to only two tokens?
I guess there should not be a grammar that needs more than two tokens of lookahead.

Also, it seems that maybe the design you chose is extensible to more than two tokens of lookahead in the future, maybe by introducing a future ThreeTokenStackOps, etc. But what lead you to this design, with two different StoreStackOps implementations? Is this for performance reason ? Is this in an attempt to prevent more upfront allocations for multiple tokens if not eventually necessary?

[atifaziz]

Looks like you beat me to opening the PR for this, but thanks for your review. Let's continue on PR #9 and I'll answer your comments there.