[proc macros]: support generic type params (#436)

* PoC support generic type params

* more annoying example

* add trait bounds for generic params in proc macros

* add compile-time test for complicated trait

* smarter trait bounds in proc macros

* add non-working example for now

* revert nits

* Update examples/proc_macro.rs

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

* Update proc-macros/src/helpers.rs

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

* add messy code but works

* cleanup

* add some simple compile check in tests

* fix doc link

* fix doc link last time

* address grumbles

* docs

* Update proc-macros/src/helpers.rs

* Update proc-macros/src/helpers.rs

* Update proc-macros/src/helpers.rs

* Update proc-macros/src/helpers.rs

* Update proc-macros/src/visitor.rs

* fix nit: | -> ||

* Update proc-macros/src/helpers.rs

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

* Update proc-macros/src/helpers.rs

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

* Update proc-macros/src/helpers.rs

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

* add issues to introduced TODOs

* generics support where clause on trait

* Update proc-macros/src/helpers.rs

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

* Update proc-macros/src/helpers.rs

* address grumbles

* add more docs

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