[Runtime] Python String container interface

[wweic]

• Provide python interface to use runtime String container.
• Migrate existing string use cases to the python interface.

[tqchen]

cc @zhiics @mbaret @comaniac @jroesch we probably needs to migrate the use of StringImm to String once the string container is introduced

[wweic]

@tqchen Could you give me pointers to codes where we can use the python string api to pass strings around? I'll use them as example to drive my implementation.

[zhiics]

@wweic, probably all places we used stringimm?

[tqchen]

I can see the following steps:

• Introduce the string container
  • We need to think about how to make the String object works like a normal python string(possibly subclass str?, I haven't dig deep)
  • At this stage users can pass and return String but need to explicitly do so
• Add automatic conversion from TVMArgValue to String, when the argument is c string pointer and bytes, automatically convert to a String container(via copy).
• Change the original usage of StringImm to use String instead
• Change automatic object conversion(tvm.convert) https://github.com/apache/incubator-tvm/blob/master/python/tvm/runtime/object_generic.py#L59 to convert python str to String object
• Remove constructor of Expr from std::string(as it will no longer be needed), we will use Array for most cases
• Add reflection support to serialize a String (so we can dump a String object to json).

[tqchen]

#5426