Array splicing

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.)

Start with issue #374 and compare its proposed functional array updates with the referenced functional struct updates and replication syntax. Resolve the syntax, type-checking, and postfix-versus-prefix questions described in the issue; done would require an agreed language design and an accepted RFC.