[REFACTOR] top - namespace for Tensor Operation DSL (#4727)

[REFACTOR]  top - namespace for Tensor Operation DSL (#4727)

* [REFACTOR] introduce top - Tensor Operation DSL.

Historically we put Tensor, Schedule and compute under the root tvm namespace.
This is no longer a good idea as the project's scope grows larger
than the tensor operation DSL.

This PR introduces top -- a namespace for tensor operational
DSL concepts such as schedule, tensor, compute.
We moved the related files to the new top subfolder.

* Move relevant files into include/tvm/top and src/top