Cleanup of old deps

[Xuanwo]

cxx-async has some really old deps, it's better to cleanup them:

cxx-async-macro

• syn 1 => 2 (And do we really need full?)
• pin-utils remove (it's part of std): https://github.com/rust-lang/pin-utils

cxx-async-example-cppcoro

• async-recursion 0.3 => 1

cxx-async-example-folly

• async-recursion 0.3 => 1

[Xuanwo]

cc @devillove084, perhaps you'd be interested in bringing them up.

[devillove084]

@Xuanwo No problem, I will sort out a new PR later.