feat: alias attribute for proc macros (#442)

* feat: alias attribute for proc macros

* implement David's suggested no more unsub field

* keep aliases outside namespace

* revert example

* fix build

* Update proc-macros/src/attributes.rs

Co-authored-by: David <[email protected]>

* grumbles: alias -> aliases

* grumbles v2: alias -> aliases

Co-authored-by: David <[email protected]>