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)
Could you clarify how it is written at present and with this proposal?