Kernel: Rework kernel-side ELF loading

ELFs are now loaded as MemoryRegions so they don't need special handling
anywhere. This also allows file backed COW optimizations to work. This
was not the case before.

This patch removes now obsolete LoadableELF and unused ELF files from
LibElf.

kernel/include/kernel/ELF.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <kernel/FS/Inode.h>
+#include <kernel/Memory/MemoryRegion.h>
+
+namespace Kernel::ELF
+{
+
+	struct LoadResult
+	{
+		bool has_interpreter;
+		vaddr_t entry_point;
+		BAN::Vector<BAN::UniqPtr<MemoryRegion>> regions;
+	};
+
+	BAN::ErrorOr<LoadResult> load_from_inode(BAN::RefPtr<Inode>, const Credentials&, PageTable&);
+
+}