Improve no-std config
Replace the no_std feature with std, and have that set by default.
Allows building for a no_std target by disabling deault features.
Make the SDL dependencies optional, and only use them if either the
sdl or bundled feature is used. Hosted targets that want to use
SDL2 must now select one of these features. (Minor version should be
bumped because of this change.)
Add a no_std target (Cortex-M0) to CI to ensure it continues to build.
Additionally, made sure it builds for UEFI target.
cargo +nightly build --no-default-features \
--target x86_64-unknown-uefi -Z build-std=core,alloc