asserts for containers

```go
// maps
it{t}.Should(a).Have().Key('xxx')
it{t}.Should(a).Have().Key('xxx').With(b)

// slices, seq
it{t}.Should(a).Contain().A(b)
it{t}.Should(a).Contain().OneOf(b)
it{t}.Should(a).Contain().AllOf(b)
```