Second crate renaming

[madsmtm]

Prerequisite of of #33.

I'm realising that using - over _ is more common (though the standard is unclear, see rust-lang/api-guidelines#29), and the documentation explicitly recommends - in -sys crates.

But more importantly, I think the objc2 prefixing is unnecessary. At least if the goal is to make upstream objc and fruity use objc-sys.

• Keep objc2
• objc2_foundation -> objc2-foundation
• objc2_sys -> objc-sys
• objc2_block_sys (see Add sys crate for blocks #49) -> block-sys
• objc2_block -> block2?
• objc2_encode -> objc2-encode
• objc2_exception -> objc-exception-sys? objc2-exception-sys? objc-try-catch-sys? Merged into objc2, see Move objc2_exception into objc2::exception #57
• objc2_foundation_derive Removed in Remove objc2_foundation_derive #56
• Change objc2_test_utils so that we don't have to publish it

[madsmtm]

This is done!