* Define the concept of "parser" in X3
Also modernizes the entry points and the most fundamental "x3::parser"
base class.
Includes overhaul on `any_parser` as it operates on iterator/sentinal
now.
Deprecate `any_parser`. The reasoning is described in the comments.
* [X3] Bump C++ version to C++23
* Set correct compilers for C++23
* Avoid recursive instantiation on `unary_parser`
* Update Ubuntu 22.04 -> 24.04
* Specify correct flags for clang++/g++ detection
* Move deprecation warning of `any_parser` to its constructor
The concept of ForwardIterator is flawed because it mixed 2 sets of concepts (value access and traversal) into 1 package.
http://www.boost.org/doc/libs/1_65_1/libs/iterator/doc/new-iter-concepts.html
It requires value_type (const)& as return type when dereference is applied, which is not mandatory in spirit parsing. A return type which is convertible to value_type is good enough. ReadableIteratorConcept and ForwardTraversalConcept should be what we need for the iterator check.
For example, the iterator of the range returned by boost::adaptors::transform(std::string, func) is normally not a ForwardIterator. But it fulfills ReadableIteratorConcept and ForwardTraversalConcept and should be able to be parsed by spirit.
