Wishlist: Make Expressions work with inequalities

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> > I see no reason to introduce scads of new functions, such as `assertEqual`, when you can write something like `assert (hold 4 == 5)` or `assert (hold 4 > 5)`.
>
> The latter doesn't work:
>
> ```m2
> i6 : hold 4 > 5
> stdio:6:8:(3): error: no method for binary operator ? applied to objects:
> -- 4 (of class Holder)
> -- ? 5 (of class ZZ)
> ```
>

Right, we'd have to make it work. Actually, we'd have to install method lookup in the d directory first, because the code for `x>y` always just runs `x?y` to see what the answer is. If we did that, then this would work:
```m2
Thing > Expression := Expression > Thing := (x,y) -> BinaryOperation {symbol >, x, y}
```

> How does `hold` know where the expression stops?

In `hold x == y`, we're comparing `hold x` to `y`. But there is a method for doing that:
```m2

i5 : code lookup (symbol ==, Expression, Thing)

o5 = ../../../../../../Macaulay2/m2/expressions.m2:490:45-490:64: --source code:
installMethod(op,Expression,Thing,(x,y) -> g(x,expression y));
| symbol class value location of symbol
| ------ ----- ----- ------------------
| f : FunctionClosure -- ... ../../../../../../Macaulay2/m2/expressions.m2:485:5-485:6
| g : FunctionClosure -- ... ../../../../../../Macaulay2/m2/expressions.m2:489:5-489:6
| op : Keyword -- == ../../../../../../Macaulay2/m2/expressions.m2:484:33-484:35
```

_Originally posted by @DanGrayson in https://github.com/Macaulay2/M2/issues/1738#issuecomment-751016854_

Start with the expression handling in expressions.m2 and the method lookup in the d directory mentioned in the issue. Trace how held expressions handle ==, then determine what is needed for inequalities such as >; done means the shown held inequality produces the intended expression and method lookup works.

Written by the indexing model from the issue text.