Make == a symmetric method

Nobody has claimed this yet.

Equality is always a symmetric relation, and I think we should implement an exception for == so that for instance the method `Vector == ZZ` will automatically also be used if `ZZ == Vector` is called.

Here is a potential implementation:
```m2
eq0 = lookup(symbol==, Thing, Thing)
Thing == Thing := (a,b) -> (
if lookup(symbol==, class b, class a)
=!= lookup(symbol==, class a, class b)
then b == a else eq0(a,b))
```
I don't like it very much, but it works:
```m2
i1 : eq0 = lookup(symbol==, Thing, Thing);

i2 : Thing == Thing := (a,b) -> (
if lookup(symbol==, class b, class a)
=!= lookup(symbol==, class a, class b)
then b == a else eq0(a,b));

i3 : 1 == vector {1,2}
stdio:2:149:(3):[1]: error: no method for binary operator == applied to objects:
1 (of class ZZ)
2
== | 1 | (of class ZZ )
| 2 |

i4 : Vector == ZZ := (v,f) -> matrix v == f;

i5 : 1 == vector {1,2}
stdio:4:34:(3):[2]: error: matrices have different shapes

i6 : 1 == vector {1}

o6 = true
```

No source files or tests are named in the issue. Start by tracing the `==` method lookup and the proposed `Thing == Thing` implementation, then reproduce the examples involving `1 == vector {1,2}` in the Macaulay2 interpreter. Done means the requested symmetric dispatch works without breaking existing equality behavior.

Written by the indexing model from the issue text.