HaveCount with special cases for 0 or 1

This is really just a nice to have.

```c#
list.Count().Should().Be(0);
list.Count().Should().Be(1);
```
are correctly simplified into:
```c#
list.Should().HaveCount(0);
list.Should().HaveCount(1);
```

It would be nice, if they could be simplified directly into the `ContainSingle` and `BeEmpty` instead.

```c#
list.Should().ContainSingle();
list.Should().BeEmpty();
```