Put methods behind feature flags

[madsmtm]

Fixes #311.

A few manual fixes are still necessary to handle structs and typedefs that use class types, and the user has to specify e.g. ["Foundation", "Foundation_NSString"] instead of just ["Foundation_NSString"], but it should be workable.

[silvanshade]

Awesome, thanks for starting on this!

[silvanshade]

I made some additions in a local branch at https://github.com/silvanshade/objc2/tree/icrate-behind-feature-flags. It's able to automatically emit the features to the icrate/Cargo.toml (see the comment at the end of the file) although it's apparently missing some cases, e.g.:

feature `AppKit_NSScrubberItemView` includes `AppKit_NSScrubberArrangedView` which is neither a dependency nor another feature

In any case, may not be the best way to do things but feel free to use any of that if it's useful.

[madsmtm]

Thanks, I started from what you had and have gotten feature emission to work now (not currently updated in-branch).

[madsmtm]

So wrt. compilation-time improvements (approximate numbers):

Label	--features=Foundation currently	--features=Foundation now	--features=Foundation_all now
debug check	~21s	~4s	~20s
debug build	~31s	~5s	~33s
release build	~30s	~4s	~26s

Tested with:

CMD=cargo +nightly build -picrate --features Foundation # or `check` or `build --release`
$CMD
cargo clean -picrate
$CMD

Which tells me that this is a definite improvement (and not even a regression when enabling many features), and it is definitely good enough for me to actually start using it in winit!