Make `Time` etc structs accessible

There is no further support for `Time`, `DateTime` and the like besides the fields given by the PackStream specification. It would be nice to have some interface to `std::chrono` in a suitable fashion.

To these, there is probably a shared common core which can be used across the different standard structs, but the just be accessible for associated functions, i.e. as part of e.g.
```rust
impl Time {
pub fn from_utc(...) { // ... }
}
```