feat(api): add new/move type aliases into api.ts

- Fn, FnAny, Pair, SEMAPHORE

packages/api/src/api.ts

@@ -4,6 +4,8 @@ export const EVENT_ALL = "*";
 export const EVENT_ENABLE = "enable";
 export const EVENT_DISABLE = "disable";
 
+export const SEMAPHORE = Symbol();
+
 /**
  * Generic 2-element comparator function type alias. Must follow this
  * contract and return:
@@ -14,6 +16,16 @@ export const EVENT_DISABLE = "disable";
  */
 export type Comparator<T> = (a: T, b: T) => number;
 
+/**
+ * A single arg function from A => B.
+ */
+export type Fn<A, B> = (x: A) => B;
+
+/**
+ * A vararg arg function to type T.
+ */
+export type FnAny<T> = (...xs: any[]) => T;
+
 /**
  * Event listener.
  */
@@ -24,6 +36,11 @@ export type Listener = (e: Event) => void;
  */
 export type Path = PropertyKey | PropertyKey[];
 
+/**
+ * A key-value pair / tuple.
+ */
+export type Pair<K, V> = [K, V];
+
 /**
  * Predicate function mapping given value to true/false.
  */