Make type check utilities

[unnonouno]

• Make an expression tree to validate types and show errors
• Add a function to check types to Function. The function is called with expression trees
• Make examples of type check

fixes #1

For example, when 4th dimension of 1st argument of an input is expected to be 2, you can write like this:

def ceck_type_forward(self, in_types):
    in_types[0].shape[3].should_be(2)

And, actually it is 1, a user can get a error message like this:

Expect: in_types[0].shape[3] == 2
Actual: 1 != 2

[delta2323]

If get_type is not called only from get_types, I think get_type should be in get_types to narrow its scope.

[unnonouno]

In offline discussion, we decided to use this style:

def ceck_type_forward(self, in_types):
    type_check.expect(
        in_types[0].shape[3] == 2,
        in_types[0].shape[2] == 1,
    )

[delta2323]

Can I write in_types[0].ndim.eval > self.axis instead? I think it is better because value of the left hand side is used afterwards as ndim.

[delta2323]

By offline discussion, I found that it is necessary to wrap self.axis in order to print variable name axis when this expectation failed.

[delta2323]

Bool is misunderstanding with bool (True or False)

[delta2323]

LGTM 👍