Text-based fancy output

```cs
Entity a = "1/2";
Console.WriteLine(a);
```
outputs, as expected, `1/2`. But what if there was some
```cs
Entity a = "1/2";
Console.WriteLine(a.FancyToString());
```
or something, which would return
```
1
---
2
```

Imagine `sqrt(2) + 1/2` as
```
/-- 1
/ 2 + ---
\/ 2
```

Start by reviewing the Entity type and the proposed FancyToString entry point, then compare the requested text layouts for fractions and square roots. Done means providing a defined formatting API that produces the illustrated multiline output for expressions such as 1/2 and sqrt(2) + 1/2.

Written by the indexing model from the issue text.