Implement diagram expansion

[NickHu]

The dual to contraction - when you make a move like the following:

| |        | |
o o   ->   o |
| |        | o
           | |

This procedure is not natural and involves making arbitrary choices

[jamievicary]

In the previous version this can be found here: https://github.com/homotopy-io/homotopy-core/blob/6683058532155e3d270c4d758c8d6d1fd67df66b/src/limit.js#L278