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.

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

CC @PaulDance

[madsmtm]

I recall having a quick look at the amount of block signatures that Apple uses a month back, and it was unfortunately several hundred, which is probably too large a compile-time hit :/.

So we'll probably have to wait for generic const expressions (see 050b502 for a test with that).

[madsmtm]

An idea from #70: Since we can generate encoding strings in const, just not generically, maybe add something like:

// block2
pub fn block_encoding_str_len(args: &[Encoding], ret: &Encoding) -> usize { ... }
pub fn block_encoding_str_array<const LEN: usize>(args: &[Encoding], ret: &Encoding) -> [LEN; u8] { ... }

// Usage
struct MyBlockEncoding;
// SAFETY: The encoding is correct.
unsafe impl ManualBlockEncoding for MyBlockEncoding {
    type Arguments = (*mut NSError,);
    type Return = i32;
    const ENCODING_CSTR: &'static CStr = {
        const LEN: usize = block_encoding_str_len(<(*mut NSError,)>::ENCODINGS, i32::ENCODING_RETURN);
        let arr = block_encoding_str_array::<LEN>(<(*mut NSError,)>::ENCODINGS, i32::ENCODING_RETURN);
        CStr::from_bytes_with_nul(&arr).unwrap()
    };
}

And maybe wrap it in a macro like:

block2::manual_encoding!(fn MyBlockEncoding(error: *mut NSError) -> i32);