Make `riegeli::Reset(dest, args...)` an effective optimization of
`dest = T(args...)` for more types and patterns:

* `riegeli::Reset(dest, std::in_place, args...)` → `dest.emplace(args...)`
* `riegeli::Reset(dest, std::in_place_type<Type>, args...)` →
  `dest.emplace<Type>(args...)`
* `riegeli::Reset(dest, std::in_place_index<index>, args...)` →
  `dest.emplace<index>(args...)`
* `riegeli::Reset(dest)` → `dest.clear()` (`begin()` and `end()` must be
  defined because this pattern would be risky for non-containers)
* `riegeli::Reset(dest, args...)` → `dest.assign(args...)`
  (`begin()` and `end()` must be defined)

This covers in particular `std::optional`, `std::variant`, `std::any`,
`absl::InlinedVector`...

Describe some caveats when resetting may differ from assignment from a newly
constructed object.

Remove overrides of `RiegeliReset()` which became redundant.

PiperOrigin-RevId: 900610370