Proper bool handling

[madsmtm]

4 real this time: Automatic conversion of bool to/from the Objective-C BOOL across message sends.

Yes, it's unclean, and yes, it couples objc2-encode to the runtime (which it will have to be anyhow to handle bitfields properly), and feels like a lot of work for a little thing, but it's simply just a huge productivity boost to never again have to worry about bool being special!

Unblocks #217, with this it suddenly becomes much more useful!

Original idea in #126, dismissed back then because of complexity.

Random notes to myself:

• Initial implementation
• Should not be allowed in extern "C" fn(bool) in add_method (EncodeArguments)
• Document special handling of bool in Ivar
• Document in general
• Figure out how to cfg guard the stuff in objc2-encode and objc-sys properly

[ArturKovacs]

This is awesome!