Introduce IdentifierSet type for readability

[tim-barry]

Problem Definition

The idiom of using a map[flow.Identifier]struct{} for a set or lookup table of Identifiers is somewhat commonly used around the codebase (117 occurrences). However, code manipulating and interacting with these is somewhat confusing and not obvious for people unfamiliar with this specific idiom of using a map to an empty struct.

Proposed Solution

By introducing an IdentifierSet type as an alias for map[flow.Identifier]struct{} alongside IdentifierList, and adding well-named methods to manipulate it, we can clarify and increase code readability. In fact, such a type already exists, but is only used in one file (node_blocklist_wrapper.go).

Definition of Done

Common operations on IdentifierSet used in multiple places should be factored into clear methods that improve readability, such as the already-existing (s IdentifierSet) Contains(id flow.Identifier) bool. No actual functionality should be changed.