AK: Add a TRY(expression) macro to simplify the unwrap-or-return pattern

The way we use classes like Kernel::KResultOr<T> and AK::Result<T, E>
makes checking for errors (and short-circuiting returns) quite verbose.

This patch adds a new TRY(expression) macro that either evaluates to
the released result of the expression if successful, or returns the
error if not.

Before:

    auto foo_or_error = get_foo();
    if (foo_or_error.is_error())
        return foo_or_error.release_error();
    auto foo = foo_or_error.release_value();

After:

    auto foo = TRY(get_foo());

The macro uses a GNU C++ extension which is supported by GCC, Clang,
Intel C++, and possibly others. It's not *ideal*, but since it makes our
codebase considerably nicer, let's try(!) it out. :^)

Co-authored-by: Ali Mohammad Pur <mpfard@serenityos.org>

AK/Try.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+// NOTE: This macro works with any result type that has the expected APIs.
+//       It's designed with AK::Result and Kernel::KResult in mind.
+
+#define TRY(expression)                    \
+    ({                                     \
+        auto result = (expression);        \
+        if (result.is_error())             \
+            return result.release_error(); \
+        result.release_value();            \
+    })