Make blocks with encoding easier to use

[madsmtm]

#636 added [Rc|Stack]Block::with_encoding, but they're pretty cumbersome to use because you have to create a helper structure, and implement ManualBlockEncoding with the correct encoding.

What's the solution when something is cumbersome? Make it automatic!

An idea would be to write a script that uses libclang, goes through all Apple's headers, on a few different targets, and generates the correct ManualBlockEncoding for the corresponding fn(A) -> Rs. That way

This would allow the user to write RcBlock::with_encoding<fn(i32) -> i32, _, _>(|x| x * x) in the vast majority of cases.

CC @PaulDance