Extend assert.Same to support pointer-like objects

[Michael0x2a]

Summary

Modify assert.Same and assert.NotSame to support comparing maps and channels by pointer equality.

Changes

1. Rename samePointers to sameReferences and generalize it so it will try comparing objects that are represented using a single pointer/reference, as opposed to comparing only literal pointers. We maintain the invariant that the two args must have the same Kind and Type.
2. Rename Test_samePointers to Test_sameReferences
3. Add test cases to Test_sameReferences, TestSame, and TestNotSame.

Motivation

Today, assert.Same and assert.NotSame support comparing only literal pointers. This PR generalizes both functions so they also support comparing maps and channels by type and pointer equality.

Why these two types? Because the Go language reference states:

• A pointer value is a reference to the variable holding the pointer base type value.
• A map or channel value is a reference to the implementation-specific data structure of the map or channel.

That is, all three types are represented by a single reference to some underlying value.

Given this, my belief is it would minimize surprise for users if we handle all three cases in the same way in Same/NotSame: we compare the objects by pointer equality, regardless if the underlying value is a user-defined object vs some implementation-specific data structure.

By this logic, I decided against modifying Same/NotSame to support comparing things like slices, which are not represented via a single reference.

Related issues

There is a related discussion https://github.com/stretchr/testify/discussions/1732, where (iiuc) the claim was made that we should not support Same/NotSame for maps because we cannot compare map keys and values for sameness in all cases.

However, I found this a somewhat surprising argument: if Same is given a pointer to some user-defined struct, it currently does not attempt to introspect and compare the contents of that struct. So, I don't see why it's necessary or expected to do the same for maps.

Hence, this PR.