Fixtype of static information for identifiers.
This is a tagged union type, introduced by fty::deftagsum.
This is the information that a static environment associates to identifiers in scope, which may be variables, constants, functions, or struct types. There are three kinds of identifier information: one for variables and constants (see var/const-sinfo); one for function (see function-sinfo); and one for struct types (see struct-sinfo).