Consider necessity of libfmt dependency

[mazunki]

From #2309:

Approving this conditionally: I want us to make 100% sure that we can't convince std::fmt to work for us, without having to add futex. If we do need futex, please post an issue to remove this dependency if / when we get to implementing or stubbing futex.

Early printing of kprintf() currently has a fixed size allocation, meaning we could potentially get rid of libfmt as a dependency. The change itself would be close to trivial. That said, #2309 (comment) showcases how libfmt allows us to do this conversion at compile-time with dynamically sized strings. This might be useful if we want to remove the fixed size from the kernel's serial console, or for other things relying on a futex before it's ready.

On top of this, libfmt facilitates adding colours and similar things to strings. We should look into whether C++ lets us do that sort of thing in a standard way.