RFC: Change syntax of subslice matching

Change syntax of subslices matching from `..xs` to `xs..` to be more consistent with the rest of the language and allow future backwards compatible improvements.

[Rendered](https://github.com/krdln/rfcs/blob/slicematching-dots/active/0000-subslice-syntax-change.md)
Maybe I shouldn't say that it is consitent, but rather that it is not inconsistent. Current syntax suggest that when matching with `..x`, `x` would be bound to a number of elements, but it is somewhat surprisingly bound to whole slice. In my previous RFC I proposed that `x` in this case should really mean a number, and that would require moving `xs` to the left of `..`. This proposal alone makes less sense, because it somewhat cut down version, but still I think proposed behaviour is less surprising than current and provides ability to make future backwards compatible changes (which would allow consistent behaviour of token after `..` meaning a number).

Unfortunatelly, as @glaebhoerl noticed, I didn't think about syntax of struct update at all.