re-export pin-project macros

https://github.com/taiki-e/pin-project, ref https://github.com/async-rs/async-std/pull/203.

Re-exporting proc macros isn't possible, so we'll have to vendor code with proper attribution and licensing.

Start by reading the referenced pin-project repository and async-std pull request 203 to understand the proc-macro limitation and the proposed approach. Identify the code that would need to be vendored and the required attribution and licensing terms. Done means the relevant macros are available through async-attributes with proper licensing.

Written by the indexing model from the issue text.