Add a subject for CharSequence

Android mostly deals with `CharSequence` instead of `String`. Many of the `StringSubject` checks rely on the API of `CharSequence` and not `String`. Is there a reason that no subject for `CharSequence` exists? It feels like both subjects could share a lot of their implementation.

At the moment, it is necessary to call `toString()` on the sequence, what isn't that pretty.
```kotlin
val sequence: CharSequence = ""
assertThat(sequence.toString()).isEmpty()
```