* [REFACTOR] Streamline Function Attr interface.

There has been quite a few recent changes that depends heavily on
the function attr interface. This PR streamlines that interface by introducing
two APIs that covers most of the usages.

- GetAttr which gets a typed object for a given key
  - HasNonzeroAttr is a quick helper that calls GetAttr to quickly check an attribute
- WithAttr that creates a new function object with the given attr
  - The API comes with copy on write optimization to avoid multiple copies
  - We deliberately pick the prefix With(instead of Set) to indicate this
    function does not mutate the original input.

On the python side:
- We allow read access via func.attrs (which is a DictAttr)
- func.with_attrs to create a new instance with updated attrs.

We also get rid of the small wrapper functions and make sure the API centered around
the GetAttr and HasNonzeroAttr interface.

This PR also changes the function construction to follow the new convention.

* Address review comments

* Address review comments

* Fix doxygen path

* [REFACTOR][PY][API-CHANGE] Remove legacy python files.

Remove legacy python files.
Use the te namespace for most of the tensor expression primitives.

- tvm.create_schedule -> tvm.te.create_schedule
- tvm.placeholder -> tvm.te.placeholder
- tvm.compute -> tvm.te.compute

* Remove top-level exposures.

- Move related files into the corresponding location as in C++
- Keep the top-level TVM API backward compatible to make minimum changes in topi

* [REFACTOR][PY][API-CHANGE] establish tvm.ir, migrate corresponding relay files.

This PR establishes tvm.ir and migrates the corresponding relay
files into the new folder.

API Change:
- relay.Module -> tvm.IRModule

* Update with ADT

* Migrate transform

* address comments

* Migrate module

* Migrate json_compact

* Migrate attrs

* Move LoweredFunc to stmt temporarily

* temp migrate container

* Finish migrate container

* [REFACTOR][PY-API] Polish tvm.runtime, tvm.runtime.module API update

This PR updates the tvm.runtime to use the new FFI style.

- Remove top-level tvm.module to avoid confusion between runtime.Module and IRModule
- API changes wrt to runtime.Module
  - tvm.module.load -> tvm.runtime.load_module
  - tvm.module.enabled -> tvm.runtime.enabled
  - tvm.module.system_lib -> tvm.runtime.system_lib
- Remove dep on api_internal from runtime.

* Update module.load in the latest API