Restructure and Add List Matchers [WIP]

### ~~WORK IN PROGRESS~~

> ~~**Note: I was rushing through this, a lot should be cleaned up, and _GASP_ I haven't written tests yet**~~

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#### Resolves #38 

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- Reorganizes `matchers/` file tree
- Adds `matchers/lists/be-empty()`
- Adds `matchers/lists/be-longer-than()`
- Adds `matchers/lists/be-shorter-than()`
- Updates `matchers/lists/contain()`
- Adds `matchers/lists/deep-contain()`
- Adds `matchers/lists/have-length-of()`

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**TODO**

- [x] Tests for `be-empty()`
- [x] Tests for `be-longer-than()`
- [x] Tests for `be-shorter-than()`
- [x] Tests for `deep-contain()`
- [x] Tests for `have-length-of()`
- [x] Clean up files