Refactor objc2_encode

[madsmtm]

Move the traits in it to objc2::encode, and make it no_std and target/platform-agnostic. I also fixed bitfields along the way.

Now, objc2_encode just contains the parsing/representation of encodings (which may be usable elsewhere, e.g. in an Objective-C compiler or something).

This is done because I've found that keeping the Encode/RefEncode/EncodeConvert traits in a separate crate than the rest of objc2 is just too much of a hassle, and a blocker for #277.

Downside is that block2 now depends on objc2 instead of just objc2-encode, which makes the compilation graph slightly worse, but eh, on the other hand objc2-encode now has no dependencies.