Experiment request: macro fragment fields

[joshtriplett]

This is a request for a lang-team experiment for macro fragment fields (RFC 3714).

As a lang-team member, I don't need a liaison, but I do need another lang-team member to second this experiment.

I'm planning to substantially rework that RFC, as part of my ongoing work making declarative macros better. In the process, I have some experimental implementation work, which I'd like to land under the auspices of a lang-team experiment. The corresponding feature gates would be marked as incomplete until the RFC is updated and approved.

The scope of this work would be:

• Adding some additional macro fragment types, such as :fn and :adt.
• Adding initial support for macro fragment fields such as ${f.name}.