RFC: Schema Coordinates (#746)

* Add Schema Coordinates RFC

* Update schema coordinates spec edit

- Make examples table
- Tweak the grammar definition

* move spec edits to seperate pr

* schema coordinates rfc: use colons for arguments

* schema coordinates rfc: fix newline

* Apply suggestions from code review

Co-authored-by: Benjie Gillam <[email protected]>

* Update rfcs/SchemaCoordinates.md

Co-authored-by: Benjie Gillam <[email protected]>

* add notes about Foo.bar.baz vs Foo.bar(baz:)

* fix broken link

* add directive example

* emoji

* capture discussion about argument syntax

* remove open question about argument syntax

* add prior usage

* Update rfcs/SchemaCoordinates.md

Co-authored-by: Benjie Gillam <[email protected]>

* Remove outdated section

Co-authored-by: Benjie Gillam <[email protected]>

Author: magicmark