_DeepMatcher should use describeMismatch instead of describe

Currently when `_DeepMatcher` encounters a `Matcher` nested within a collection and the match fails, it uses the `describe` method to describe the mismatch rather than the `describeMismatch` method.

This is best demonstrated with an example. Consider the following expectation.

```dart
expect('foo', hasLength(4));
```

It fails with the following message which we can all agree is quite helpful.

```
Expected: an object with length of <4>
Actual: 'foo'
Which: has length of <3>
```

Meanwhile, observe what happens when we nest the value and matcher in a collection.

```dart
expect(['foo'], [hasLength(4)]);
```
**Current output**
```
Expected: [>]
Actual: ['foo']
Which: does not match an object with length of <4> at location [0]
```

While the message here is *alright*, it's not quite as good as before, and it's not useful if the actual value's `toString()` doesn't make it clear why it didn't match.

**Desired output**
```
Expected: [>]
Actual: ['foo']
Which: has length of <3> at location [0]
```
The ideal output would be akin to that of the non-nested expectation, making use of the expected matcher's `describeMismatch` method.

Locate `_DeepMatcher` and trace the nested `Matcher` failure path. Reproduce the issue with `expect(['foo'], [hasLength(4)])`, then add a regression test showing that the nested mismatch reports the actual length and location [0], matching the desired output.

Written by the indexing model from the issue text.