[API] Prefetch schedule supported (#258)

* prefetch interface added

* prefetch python comments modified. prefetch info data structure maintained.

* start injecting prefetches. first step (domain touch) implemented.

* domain touch tested.

* Prefetch ir_mutator and ir_visitor dispatch registered.

* modify domain touched from passing a func_ref to passing a tensor

* modify domain touched from passing a func_ref to passing a tensor

* modify Tensor copy to Tensor ref

* temp commit for rebase

* debug info removed, typo fixed, ready to rebase

* prefetch flatten test add!

* roll back builtin functions to side effect functions

* lint error fixed!

* add cache line size to storage flatten argument

* forgot modifications add

* change code style to dmlc-like; get rid of can_prove, use manually compute instead

* python lint error fixed

* modify instrinsic name to pass tests

* [TEST] get rid of str(), replace them by accessing attributes

* change map to list comprehension

* redundant numpy import removed