Add concat function (#2640)

src/prelude/array.ts

@@ -6,6 +6,10 @@ export function count<T>(x: T, xs: T[]): number {
 	return countIf(y => x === y, xs);
 }
 
-export function intersperse<T>(sep: T, xs: T[]): T[] {
-	return [].concat(...xs.map(x => [sep, x])).slice(1);
+export function concat<T>(xss: T[][]): T[] {
+	return ([] as T[]).concat(...xss);
+}
+
+export function intersperse<T>(sep: T, xs: T[]): T[] {
+	return concat(xs.map(x => [sep, x])).slice(1);
 }