Rewrite #[fastout]

[qryxip]

Fixes #8.

1. Migrate to syn v1.

2. Bump the MSRV to 1.42.0.

3. The new fastout will be like this:

   let __proconio_stdout = ::std::io::stdout();
   let mut __proconio_stdout = ::std::io::BufWriter::new(__proconio_stdout.lock());
   
   // proc-macros can produce `macro_rules!` since Rust 1.40.0
   
   #[allow(unused_macros)]
   macro_rules! print {
       ($($tt:tt)*) => {
           <::std::io::BufWriter<::std::io::StdoutLock<'_>> as ::std::io::Write>::write_fmt(
               &mut __proconio_stdout,
               format_args!($($tt)*),
           )
           .unwrap()
       };
   }
   
   #[allow(unused_macros)]
   macro_rules! println {
       () => {
           <::std::io::BufWriter<::std::io::StdoutLock<'_>> as ::std::io::Write>::write_all(
               &mut __proconio_stdout,
               b"\n",
           )
           .unwrap()
       };
       ($fmt:literal $($tt:tt)*) => {
           <::std::io::BufWriter<::std::io::StdoutLock<'_>> as ::std::io::Write>::write_fmt(
               &mut __proconio_stdout,
               format_args!(::std::concat!($fmt, "\n") $($tt)*),
           )
           .unwrap()
       };
   }
   
   let __proconio_res = {
       // AS IS
       println!("Hello!");
   
       // `print!`/`println!` in closures still produce explicit errors
       //thread::Builder::new()
       //    .stack_size(1024usize.pow(3))
       //    .spawn(|| println!("Hi"))
       //    .unwrap();
   };
   <::std::io::BufWriter<::std::io::StdoutLock> as ::std::io::Write>::flush(
       &mut __proconio_stdout
   ).unwrap();
   return __proconio_res;

   The new implementation does not replace qualified macros i.e. std::print!. std::println!, ::std::print! and ::std::println!. So this change may be breaking.

[statiolake]

Looks good to me, including the breaking change about std::println!. Usually, the std:: modifier is rarely used, and if it is still explicitly stated by the user, I think it is safe to assume that the user really wants to call the println! of std. Thanks!