Update Magnum to latest & fix various minor buildsystem issues

[mosra]

Motivation and Context

Changes that aren't really related to the upcoming second batch renderer PR, thus submitting them separately.

Regular update of Magnum submodules:

• Adapting to a minor API deprecation (BoolVector -> BitVector, for consistency with new BitArray containers and because it's one character less to type)
• Using now-builtin std::hash specialization for Containers::String
• No other changes worth mentioning, the major piece of work done lately isn't published yet

And some minor fixes. For faster iterations I'm having a second build directory that uses a system-installed Magnum and has various features disabled, and various code rot prevented that from working:

• Various CMake updates that are needed if Magnum is external (and are no-op if it's a subproject)
• Making a build without Bullet work again
• Making a build with dynamic Magnum work again

And finally, certain bindings code used a private header from magnum-bindings. It's private so I don't guarantee any forward compatibility, I don't want anybody to use it 😝, and it doesn't even get installed which causes a build failure if Magnum is external. I made a copy of that utility directly inside Habiat and the bindings code now uses that instead.

How Has This Been Tested

Builds locally.

Types of changes

• Docs change / refactoring / dependency upgrade

[Skylion007]

Using now-builtin std::hash specialization for Containers::String

Nice.

[Skylion007]

Hmm... "$SRC_DIR/src/esp/bindings/GfxBindings.cpp:15:10: fatal error: 'esp/bindings/EnumOperators.h' file not found"

[mosra]

Lol, sorry. Fixed.

[Skylion007]

Now clang-format is complaining a bit.

[mosra]

Fixed.

(Note to self: don't open PRs at 8 PM.)