Refactor the code to make it more scalable

[ShangkunLi]

Refactor the code to make it easier to add new dialects/passes. There mainly changes:

1. Add the include/NeuraPasses.td file to easily define new passes. Remember to add a let constructor = ... in the file. Also remember to declare the constructor function in NeuraPasses.h.
2. Move the conversions into the lib/Transforms directory.
3. You don't need to register the pass one-by-one in mlir-neura-opt.cpp. Just use the mlir::neura::registerPasses();

[tancheng]

Thanks for the cleanup @ShangkunLi, my main concern is about Conversion and Transformations. Thanks~!