Equal (`=`)

`= 𝕩`: Rank

Returns the number of dimensions in `𝕩`.

```
  = 0
0
  = 3⥊0
1
  = 3‿3⥊0
2
  3‿3‿3 ⥊ ⟨⟨0⟩⟩
3
```

`𝕨 = 𝕩`: Equal To

[Pervasive.](https://mlochbaum.github.io/BQN/doc/arithmetic.html#pervasion)
```
  1 = 3
0
  2‿3‿0 = 3‿1‿0
⟨ 0 0 1 ⟩
  'a' = 'a'
1
```