Decouple the evaluation cache from the Nickel interpreter

[yannham]

As part of the incremental evaluation effort #1589, the first step is to try to decouple as much as possible the interpreter from the specific notion of thunk. Thunks are an implementation device of the call-by-need evaluation strategy (often called lazy evaluation). Call-by-need is, itself, one specific strategy for caching call-by-name evaluation (we choose to cache every 0-ary let-bound expression, and only them).

The whole idea behind our cached call-by-name is to make the interpreter agnostic to the caching and evaluation strategy. We should refactor the current interpreter such that the current notion of thunks is replaced by an abstract cache index type (to be understand as a kind of pointer, but for which we need to go through the cache), and try to remove all explicit references to thunks. This effort has already been started, but isn't finished yet, see #1220.