WIP: Add keyby iterator

[segeljakt]

This PR adds support for a keyby iterator which can be used as:

let s = streamappender[i32];
...
let s = result(s);
for(keyby(s, |e| u64(e)), appender[i32], |b,k,v| merge(b,v));

|e| u64(e) is a function which takes an element and returns a corresponding key, and is used for partitioning elements. For example, [1,2,3,3] would in this case be partitioned into: [1], [2] and [3,3]. The body of the for loop then iterates over each key-value, where k is the key and v is the value.

[segeljakt]

I can squash before merging

[Bathtor]

I think we should also have a typing rule stating that data.type for keyby iterators must be stream, since they make little sense on an array, for example, where, as you pointed out, groupmerger can already achieve this effect.

Furthermore, if we want to be able to use this, we might want to introduce some way to generate hashes efficiently. Right now we can essentially cast numbers to u64 and add them up. We could add some syntactic sugar, i.e. a hash operator that generates casts and sums, as well as magic numbers for structs for example. Then the keyby-function would essentially look like |e| hash({e.$1, e.$2}), for example, instead of |e| 89 + ((u64)e.$1) + ((u64)e.$2) or whatever.

[segeljakt]

Ok, good points. I'll add some more commits

[segeljakt]

I think it is best if I continue with hash in a separate PR #3.