Kernel: Add KResult and KResultOr<T> classes.

The idea here is to combine a potential syscall error code with an arbitrary
type in the case of success. I feel like this will end up much less error
prone than returning some arbitrary type that kinda sorta has bool semantics
(but sometimes not really) and passing the error through an out-param.

This patch only converts a few syscalls to using it. More to come.

Kernel/SyntheticFileSystem.cpp

@@ -311,8 +311,7 @@ size_t SynthFSInode::directory_entry_count() const
     return m_children.size() + 2;
 }
 
-bool SynthFSInode::chmod(mode_t, int& error)
+KResult SynthFSInode::chmod(mode_t)
 {
-    error = -EPERM;
-    return false;
+    return KResult(-EPERM);
 }