The second template parameter, defaulted to void, was only used for
SFINAE with enable_if. Now that all uses of enable_if are gone replaced
by concepts, this parameter does not serve any purpose anymore.
This commit drops the KeyTraits in favor of a std::any-like solution.
Basically, we type erase any key the user wants to employ, just exposing
a virtual version of the destructor, a comparison operator and a clone
primitive.
This commit:
* Drops `KeyTraits::toString`: if needed, use `getNameFromYAMLScalar`.
* Makes many methods in TupleTree.h return `nullptr` or `std::optional`
in order to gracefully handle failures.
* Provides `KeyTraits` specializations for integral types and tuple-like
composed by types providing `KeyTraits`.
* Introduces `CompositeScalar`, which enables tuple-like objects to be
YAML-serializable scalars by joining the YAML-serialization of its
members through a customziable character.
* Implements `PathMatcher`, a very simple "regular expression" mechanism
for paths on tuple trees.
* Introduce testing for the Model.
These data structures are substitutes for a `std::map<Key, Value>` where
`Key` is embedded in `Value`. Their main goal is to be serializable in a
YAML sequence while preserving the order enforced by the key.
`MutableSet` is implemented using a map.
`SortedVector` is implemented using a sorted vector.
Pietro Fezzardi