This PR moves the codegen related code into the target folder,
as they are target specific functionalities.

We also adopt the term "compiler driver" in common compiler infra
such as rust, GHC and clang.
As a result, build_module is moved into the driver folder.

TIR is the new namespace for low-level IR
for tensor-level optimizations and loop transformations.

This PR establishes the namespace and files.

- lowered_func.h,buffer.h,data_layout.h -> tir/buffer.h,tir/data_layout.h,tir/lowered_func.h
- ir.h -> tir/expr.h, tir/stmt.h
- ir_functor_ext.h -> tir/expr_functor.h, tir/stmt_functor.h

* [REFACTOR][IR] Variable -> VarNode

* [REFACTOR][IR] Add/Sub/Mul/Div -> AddNode/SubNode etc.

* [REFACTOR][IR] Min/Max/FloorDiv/FloorMod -> MinNode/MaxNode etc.

* [REFACTOR][IR] EQ/NE/LT/LE/GT/GE/Select -> EQNode/NENode etc.

* [REFACTOR][IR] Add Node suffix to Select/Call/Load/Ramp/Shuffle/Let

* [REFACTOR][IR] Add node suffix to IntImm/UIntImm/FloatImm/StringImm

* [REFACTOR][IR] Add Node suffix to Any, AttrStmt, AssertStmt

* [REFACTOR][IR] Add Node suffix to Store/Provide/Allocate/Free

* [REFACTOR][IR] Add Node suffix to ProducerConsumer

* Fix lint

* style updates, test fixes

* [RUNTIME][DSO] Improve TVMBackendPackedCFunc to allow return value.

Previously the signature of LibraryModule's PackedFunc does not support return value.
This wasn't a limitation for our current usecase but could become one
as we start to generate more interesting functions.

This feature also start to get interesting as we move towards unified
object protocol and start to pass object around.
This PR enhances the function signature to allow return values.

We also created two macros TVM_DLL_EXPORT_PACKED_FUNC and TVM_DLL_EXPORT_TYPED_FUNC
to allow manual creation of functions that can be loaded by a LibraryModule.

Examples are added in apps/dso_plugin_module.
The change to TVMBackendPackedCFunc is backward compatible,
as previous function will simply ignore the return value field.

* address review comments

dtype.h -> runtime/data_type.h

Changes:
- Rename all old reference of tvm::Type to DataType
- ExprNode.type -> ExprNode.dtype
- Expr.type() -> Expr.dtype()
- Change Expr related functions to expr_operator.
  - DataType::min() -> min_value(DataType)
  - DataType::max() -> max_value(DataType)
- Move type constructor Int, UInt, Float, Handle, Bool into DataType.
  - Int(bits) -> DataType::Int(bits)
  - UInt(bits) -> DataType::UInt(bits)

* [LINT] Improve the check tool to handle ASF copyright message.

* [LINT] Remove unnecessary copyright message as per ASF requirement.

* Fix codegen hybrid

* [LINT] Broaden license checks to include html, xml

* [LINT] Fix rest of the files

* Fix notice

* [LINT] Improve check file type error message

* [HEADER] ASF header dir=include

* [HEADER] ASF Header dir=src

* [HEADER] ASF Header -dir=python

* [HEADER] ASF header dir=topi

* [HEADER] ASF Header dir=nnvm

* [HEADER] ASF Header -dir=tutorials

* [HEADER] ASF Header dir=tests

* [HEADER] ASF Header -dir=docker

* fix whitespace

* [HEADER] ASF Header -dir=jvm

* [HEADER] ASF Header -dir=web

* [HEADER] ASF Header --dir=apps

* [HEADER] ASF Header --dir=vta

* [HEADER] ASF Header -dir=go

* temp

* [HEADER] ASF Header --dir=rust

* [HEADER] Add ASF Header --dir=cmake

* [HEADER] ASF Header --dir=docs

* [HEADER] Header for Jenkinsfile

* [HEADER] ASF Header to toml and md

* [HEADER] ASF Header to gradle

* Finalize rat cleanup

* Fix permission

* Fix java test

* temporary remove nnvm onnx test

* [SCHEDULE] Add group, refactor thread bind api.

* fix doc

* fix g++-4.8

* More testscase

* Remove graph context from fix pt analysis

* [LANG/GPU] Cross Thread Reduction.

* Fix doxygen error

* Upgrade verilog testcase to new one

* [CODEGEN] Refactor common codegen, Verilog Codegen

* fix make

* fix mk

* update enable signal

* change function name to at neg edge

* Move test to correct place