Explicitly bidirectional pattern synonyms

[Dr. ERDI Gergo]

On Tue, 8 Jul 2014, Dr. ERDI Gergo wrote:

> I've spent the last couple evenings implementing explicitly-bidirectional 
> pattern synonyms, partly while waiting for someone to help out with #9023. 
> Explicitly-bidirectional pattern synonyms are described in #8581.

Please note I've uploaded an implementation of that to 
wip/pattern-synonyms, and it's ready for merging into master after someone 
(maybe SPJ after he comes back?) reviews it.