karm-async: Require a implementation for building.

src/impls/impl-posix/manifest.json

@@ -12,6 +12,7 @@
     "provides": [
         "karm-base-impl",
         "karm-logger-impl",
-        "karm-sys-impl"
+        "karm-sys-impl",
+        "karm-async-impl"
     ]
 }

src/libs/karm-async/manifest.json

@@ -4,6 +4,7 @@
     "type": "lib",
     "description": "Coroutine-based asynchronous programming library",
     "requires": [
-        "karm-base"
+        "karm-base",
+        "karm-async-impl"
     ]
-}
+}

src/libs/stdcpp-shim/coroutine

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <cstddef>
+
 namespace std {
 
 template <typename ReturnType, typename... Args>
@@ -23,6 +25,9 @@ struct coroutine_handle<void> {
     coroutine_handle()
         : _frame{nullptr} {}
 
+    coroutine_handle(std::nullptr_t)
+        : _frame{nullptr} {}
+
     explicit operator bool() {
         return _frame;
     }

src/libs/stdcpp-shim/cstddef

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <stddef.h>
+
+namespace std {
+
+using nullptr_t = decltype(nullptr);
+
+} // namespace std