Integrate numerical label IDs to Yul

[clonker]

With this change, nodes in the Yul AST are no longer referred to by string name (or more precisely: statically managed repository of strings and referenced by hashes thereof) but by numerical ID. The numerical ID belongs to an immutable ASTLabelRegistry, in which it is associated to a string label. In turn, the registry is owned by the yul::AST.

During, e.g., optimization it is possible to generate new LabelIDs, which not yet have an associated label in a registry but encode a parent label (which may be empty) using the yul::LabelIDDispenser. Newly generated IDs are only valid with respect to their generating instance of LabelIDDispenser.
At the end of optimization, when f.ex. serialization back into a stringified form is required, the LabelIDs currently contained in the AST are assigned new labels by the yul::LabelIDDispenser, generating a new instance of ASTLabelRegistry.

This PR removes YulString, yul::NameSimplifier, yul::NameDispenser, yul::VarNameCleaner and replaces them by the LabelID and related utilities.

There is a special case of ghost nodes. These are introduced in CFGs to map switch statements to sequences of if-else blocks but do not live in the actual AST, just the CFG. To this end, a CFG also holds an instance of ASTLabelRegistry and manages the LabelID -> string_view transformation itself. An ASTLabelRegistry itself does not know how to stringify the special ghost nodes.

Depends on #15838.