Proposal for enhancement of phlex::experimental::identifier #878

The class phlex::experimental::identifier (from now on, just identifier) is in some contexts where it is critical that the string never be empty, and in a few where it is allowable for the string to be empty.

For example, in phlex::detail::product_specification, the member suffix_ is an identifier that is allowed to be empty.
In phlex::experimental::algorithm_name (which is the unique identifier for a node in the graph), the member plugin_ is arguably allowed to be empty, while the member algorithm_ is not allowed to be empty.
In phlex::experimental::layer_path, the member layer_path_ is a std::vector<identifier> none of which should be empty.

The non-emptiness is not always currently enforced at runtime.
There has also been an intent to enforce other restrictions on identifiers, e.g. issue #343 to ensure that identifiers when turned into a std::string are valid C++ variable names.

I propose a change that would make the non-empty nature of identifier be a class invariant for the type, accompanied by the introduction of a new type that can have the state that represents either an identifier or an empty string; this type could have type aliases of suffix_t and plugin_name_t.

This would allow use of identifier as a type in code that would then never need to check if the "string" is empty, for example if emitting the stringified version as a label in a JSON/YAML/FHiCL document, etc., while allowing suffixes and plugin names to be empty when desired.