Array splicing #374
Description
Analogously with functional struct updates, we could also allow "functional array updates", or in other words splicing of fixed-length arrays:
let a: [int, ..3] = [1, 2, 3];
let b = [..a, ..a, ..a];
Here b: [int, ..9]. Unlike FSU, there can be any number of array splices in any positions.
This has some syntactic overlap with replication:
let c = ['c', ..10];
let d = ['d', ..x];
but even the latter case can always be disambiguated based on the type of x (it's either an integer or an array; it can't be both).
(Also, we may want postfix rather than prefix .. in this case, removing the overlap completely. But we should also want that for FSU.)