* [REFACTOR][IR] Unify IntImm and UIntImm

This PR unifies UIntImm and IntImm to simplify the codebase.
Unsigned integer constants will also be stored as IntImm.

For uint constant that does not fit into int64(rare case), we introduced
an intrinsic tvm_big_uint_imm to construct such intgers by its
lower and higher 32bits.

* [REFACTOR][IR] Remove UIntImm to use IntImm

* rename big->large

* [REFACTOR][IR] tvm::Expr -> PrimExpr(Primitive Expr)

As part of unified IR, we will need to unify relay::Expr
and the current tvm::Expr under the same base type.

From the techinical point of view. tvm::Expr is a "primitive"
expression that only contains POD types and handles and does
not do life-cycle management.

This PR renames Expr->PrimExpr to clarify that.
We will send a subsequent PR to introduce the base expr class.

* Remove legacy VarExpr and ExprHash/Equal

* [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

* i think it works for now?

* fix lint

* fix 2/3 compat

* fix py2 again

* fine, i gave up

* a preliminary version is done?

* we no longer need the redundant hybrid/api.py

* support assert stmt

* cast supported

* intrin -> runtime; util is mainly in charge of compilation time

* assert statement

* fix python lint

* fix cpp lint

* on the way to module

* rollback .cc

* fix typo, no direct expose then

* @vinx13 ceil is added i guess?

* wip...

* temp commit

* fix import

* i preliminary version is done?

* on the way to build hybrid module

* nearly fixed...

* dumped python are equiv as original python

* on the way to bootstrap

* cpu bootstrap done

* bootstrap!

* fix lint

* fix doc

* resolve some review concerns

* support load/save

* fix lint

* thanks to xqdan fixed my typo

* fix build, make dump non-optional

* add vthread

* jesus why i added this