Add array and monad lib files.

dm_lib/std/core/array.dm

@@ -1,5 +1,12 @@
 ns std::core
 
-decl pub int Array {
+decl pub int Array<T> : Monad {
     length: Int
 }
+
+pub mod array {
+    // Usage:
+    // let int_array = array::of(1, 2, 3)
+    // assert_eq(3, int_array.length)
+    decl pub fn of<T>(ts: ...T): Array<T>
+}

dm_lib/std/core/monad.dm

@@ -0,0 +1,4 @@
+pub hkt Monad[T<A>] {
+    fn <B> map(m: fn (from: A) => B): T<B>
+    fn <B> flat_map(m: fn (from: A) => T<B>): T<B>
+}