Ability to specify Map/Record key types/formats/patterns

I had asked for advice in this discussion #1626 and got pointed to creating an issue.

If you use

`alias MyMap = Record;`

You can't say anything about the keys. They are just `string` as far as I can tell.

I wanted to be able to put something like `@pattern("[a-f0-9]{10}")` on my keys.
Could also imagine: `@format("uuid") `

Hope that is detailed enough.