Everywhere: Prevent risky implicit casts of (Nonnull)RefPtr

Our existing implementation did not check the element type of the other
pointer in the constructors and move assignment operators. This meant
that some operations that would require explicit casting on raw pointers
were done implicitly, such as:
- downcasting a base class to a derived class (e.g. `Kernel::Inode` =>
  `Kernel::ProcFSDirectoryInode` in Kernel/ProcFS.cpp),
- casting to an unrelated type (e.g. `Promise<bool>` => `Promise<Empty>`
  in LibIMAP/Client.cpp)

This, of course, allows gross violations of the type system, and makes
the need to type-check less obvious before downcasting. Luckily, while
adding the `static_ptr_cast`s, only two truly incorrect usages were
found; in the other instances, our casts just needed to be made
explicit.

Kernel/FileSystem/ProcFS.cpp

@@ -55,7 +55,7 @@ KResult ProcFS::initialize()
     auto root_inode = ProcFSComponentRegistry::the().root_directory().to_inode(*this);
     if (root_inode.is_error())
         return root_inode.error();
-    m_root_inode = static_cast<NonnullRefPtr<ProcFSDirectoryInode>>(root_inode.release_value());
+    m_root_inode = static_ptr_cast<ProcFSDirectoryInode>(root_inode.release_value());
 
     return KSuccess;
 }

Kernel/FileSystem/SysFSComponent.cpp

@@ -61,12 +61,12 @@ SysFSDirectory::SysFSDirectory(StringView name, SysFSDirectory const& parent_dir
 {
 }
 
-NonnullRefPtr<Inode> SysFSDirectory::to_inode(SysFS const& sysfs_instance) const
+NonnullRefPtr<SysFSInode> SysFSDirectory::to_inode(SysFS const& sysfs_instance) const
 {
     return SysFSDirectoryInode::create(sysfs_instance, *this);
 }
 
-NonnullRefPtr<Inode> SysFSComponent::to_inode(SysFS const& sysfs_instance) const
+NonnullRefPtr<SysFSInode> SysFSComponent::to_inode(SysFS const& sysfs_instance) const
 {
     return SysFSInode::create(sysfs_instance, *this);
 }

Kernel/FileSystem/SysFSComponent.h

@@ -26,7 +26,7 @@ public:
     virtual RefPtr<SysFSComponent> lookup(StringView) { VERIFY_NOT_REACHED(); };
     virtual KResultOr<size_t> write_bytes(off_t, size_t, UserOrKernelBuffer const&, FileDescription*) { return EROFS; }
 
-    virtual NonnullRefPtr<Inode> to_inode(SysFS const&) const;
+    virtual NonnullRefPtr<SysFSInode> to_inode(SysFS const&) const;
 
     InodeIndex component_index() const { return m_component_index; };
 
@@ -45,7 +45,7 @@ public:
     virtual KResult traverse_as_directory(unsigned, Function<bool(FileSystem::DirectoryEntryView const&)>) const override;
     virtual RefPtr<SysFSComponent> lookup(StringView name) override;
 
-    virtual NonnullRefPtr<Inode> to_inode(SysFS const& sysfs_instance) const override final;
+    virtual NonnullRefPtr<SysFSInode> to_inode(SysFS const& sysfs_instance) const override final;
 
 protected:
     explicit SysFSDirectory(StringView name);

Kernel/Graphics/GraphicsManagement.cpp

@@ -130,7 +130,7 @@ UNMAP_AFTER_INIT bool GraphicsManagement::determine_and_initialize_graphics_devi
     // Note: If no other VGA adapter is attached as m_vga_adapter, we should attach it then.
     if (!m_vga_adapter && PCI::is_io_space_enabled(address) && adapter->type() == GraphicsDevice::Type::VGACompatible) {
         dbgln("Graphics adapter @ {} is operating in VGA mode", address);
-        m_vga_adapter = adapter;
+        m_vga_adapter = static_ptr_cast<VGACompatibleAdapter>(adapter);
     }
     return true;
 }

Kernel/Syscalls/purge.cpp

@@ -27,7 +27,7 @@ KResultOr<FlatPtr> Process::sys$purge(int mode)
                 if (vmobject.is_anonymous()) {
                     // In the event that the append fails, only attempt to continue
                     // the purge if we have already appended something successfully.
-                    if (!vmobjects.try_append(vmobject) && vmobjects.is_empty()) {
+                    if (!vmobjects.try_append(static_cast<Memory::AnonymousVMObject&>(vmobject)) && vmobjects.is_empty()) {
                         result = ENOMEM;
                         return IterationDecision::Break;
                     }