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
}

[rpominov]

I've added support of new protocol and old one also still supported. Will release a patch version with this changes a bit later. Also going to remove support of old protocol in v2.0.0.

Thanks!