groot: robust interop w/ C++ std::set and std::map

[Sebastien Binet]

right now, we represent std::set<T> and std::map<K,V> (and their multiX and unordered_X equivalents) as map[T]struct{} and map[K]V.

however, e.g. std::set<std::vector<T>> breaks this conversion: map[[]T]struct{} isn't a valid Go type ([]T isn't hashable).
same issue for, say, std::map<std::vector<T>, V>.

there are 2 use cases:

• read a ROOT file produced with C++/pyroot and thus generate equivalent Go types to hold these data
• write a ROOT file from groot (so with hopefully already valid Go types)

obviously, the latter isn't impacted by this issue.
to tackle the former, a possible avenue would be to implement dedicated types mimicking C++ semantics:

type Map[K, V any] struct { ... }
type Set[T any] struct { ... }
type MultiMap[K, V any] struct { ... }
type MultiSet[T any] struct { ... }
type UnorderedMap[K, V any] struct { ... }
type UnorderedSet[T any] struct { ... }
type UnorderedMultiMap[K, V any] struct { ... }
type UnorderedMultiSet[T any] struct { ... }

ultimately, this might be better resolved by implementing on-the-fly conversions between on-disk and in-memory representations:

• set<T> to/from []T
• map<K,V> to/from []pair[K,V]