More support for Array and Slice

[ranjitjhala]

Fixes #184 maybe?

The main big change is factoring Array and Slice as BaseTy to allow refining with size.

[nilehmann]

My initial reaction was that indexing arrays by the length would cause inconsistencies as you could write an array where the indexed length is different than the const length

#[flux::sig(fn([i32; _][2]))]
fn myarr(a: [i32; 1]) {
    a[1]
}

But if array literal are the only way to construct arrays then I think this should be ok and it just declaring an inhabited type. However, not connecting both lengths makes code cumbersome. For example, it makes it impossible to use the result of a function like https://doc.rust-lang.org/std/primitive.u32.html#method.to_be_bytes without explicitly checking the length, even though you already know the length is 4. Ideally, arrays shouldn’t be indexed but rather we the const length should be automatically reflected into the logic when needed, i.e., the typing rule for Len should be something like

|- a: [T; n]         reflect(n) = a
———————————————————————————--———-——-
          |- Len(a): usize[a]

I’m ok with leaving the implementation like this for now as doing this reflection requires further design.

[nilehmann]

Suggested change

// Ty::indexed(BaseTy::Array(ty, c), RefineArgs::empty())

[ranjitjhala]

Merging this in, let me make a separate issue for the Array thing.