Update transduce to Official Transformer Protocol

[kevinbeaty]

Change transducers implementation to support "Official Transformer Protocol" discussed in cognitect-labs/transducers-js#20 . Both transducers.js and transducers-js have been updated to support it.

Summary of changes:

// anywhere you use a transformer
// (including transducer implementations)
var xf = {
   init: function()
   step: function(acc, item)
   result: function(acc)
}
// becomes
var xf = {
   '@@transducer/init': function()
   '@@transducer/step': function(acc, item)
   '@@transducer/result': function(acc)
}
// reduced wrapper object changes from 
{
   value: acc,
   __transducers_reduced__: true
}
// to
{
   '@@transducer/value': acc,
   '@@transducer/reduced':true
}

[briancavalier]

Awesome, thanks for the heads up, @kevinbeaty! Do you think it's worth supporting both protocols for some period of time? It looks like it'd be easy.

I should actually have time to get this done I've the weekend.

[kevinbeaty]

@briancavalier I don't know if it's worth it as transducers-js, transducers.js and others already published the updated API without backwards compatibility. It's technically a breaking change though, so it's really your call

[briancavalier]

@kevinbeaty Ok, cool. I think I'll support both, but only for a short period. Pull request coming soon.