Have bridge code use `origin_node_->attrs.parsed` instead of `origin_node_->attrs.dict`

[cconvey]

E.g., in src/ngraph/ngraph_graph.cc:

#include "../../src/operator/nn/deconvolution-inl.h"
...
const auto & op_params = dmlc::get<mxnet::op::DeconvolutionParam>(orig_node_->attrs.parsed);
... op_params.stride ...

We can use this approach for all new bridge code, and optionally revise existing bridge code to use this approach.

Pros:

• We avoid re-parsing strings to more explicit C++ types.
• We avoid duplicating other MXnet code that sets defaults, which is a potential source of bugs.
• Possibly more idiomatic, so it might reduce resistance to upstreaming.

Cons:

• We'll need one extra #include to access each op's ...Params struct.

[mbrookhart]

I think we need to do this, but we also need to refactor the ops into multiple files to isolate the includes at the same time.