- Use consistent constructor style to construct objects.
- Move env_func to ir as it is mainly used to construct IRs.
- Make docs consistent.

This PR migrates relay's Op into the ir folder.
Op and its registry provides an useful mechanism to
store any attribute meta-data of an operator include
function signatures, lowering rules, side effect etc.

These features are not only useful for Relay, but also needed in the low-level IR.
At the current moment, intrinsic functions in the low-level IR are simply
represented by a string. This means we cannot type-check the low-level IR
when the type does not meet the constraint, nor can we obtain further
information such as side-effect and read write relation of these intrinsics
wrt to arguments.

Op will be used as the way to handle primitive ops(in DL terminology)
(builtin intrinsics or in compiler terminology).
We will perform follow-up refactors to make low-level CallNode
take Op as the function argument.

This PR moves a few base types from relay and low-level Expr into the ir sub-folder.
These classes will serve as a common type system across the stack.

Rationale:

- PrimExpr for low-level expressions
- RelayExpr for advanced features, including Function definition.
- Introduce BaseFunc to host all functions, including future PrimFunc(low-level expr functions, subject to discussion).

This is a minimum change we can do to unify the classes into a common hierarchy.
The main data structure that are variant specific will still be kept in the sub-namespaces.
We only include classes that is needed to allow a common Module class.
- BaseFunc
- GlobalVar
- Type definition part of ADT

We will only need the BaseFunc and their checked_type to decide the calling convention
across the function variants.

This PR moves a few base types from relay to the ir sub-folder.
These types will serve as a common type system across the stack.

Notably, we want to be able to use the same FuncType for all function signatures.
I tried to make a minimum move to bring the necessary dependencies for a FuncType.
We can discuss what additional things we want to move as a follow-up.

Notably, because the TensorType will have a dependency on low-level Expr,
we will need to break the type.h into two files and introduce a
tensor_type.h(or leave them in relay for now).