add Defaultable trait

Signed-off-by: Max Brylski <helehex@gmail.com>

stdlib/src/builtin/value.mojo

@@ -99,6 +99,40 @@ trait Copyable:
         ...
 
 
+trait Defaultable:
+    """The `Defaultable` trait describes a type with a default constructor.
+
+    Implementing the `Defaultable` trait requires the type to define
+    an `__init__` method with no arguments (other than self):
+
+    ```mojo
+    struct Foo[default: StringLiteral](Defaultable):
+        var s: String
+
+        fn __init__(inout self):
+            self.s = default
+    ```
+
+    You can now construct a generic `Defaultable` type:
+
+    ```mojo
+    struct Pair[T1: Defaultable, T2: Defaultable]:
+        var bleep: T1
+        var bloop: T2
+
+        fn __init__(inout self):
+            self.bleep = T1()
+            self.bloop = T2()
+
+    var pair = Pair[Foo["Aigis"], Foo["Max"]]()
+    ```
+    """
+
+    fn __init__(inout self):
+        """Create a default instance of the value."""
+        ...
+
+
 trait CollectionElement(Copyable, Movable):
     """The CollectionElement trait denotes a trait composition
     of the `Copyable` and `Movable` traits.