Add field accessing and indexing of arguments to `format!`

[tikue]

This would be nice if for no other reason than to reach feature parity with Python's str.format(), in which one can write things like:

>>> "{foo[0]} and {foo[1]} went up the hill".format(foo=['jack', 'jill']) 'jack and jill went up the hill'

[huonw]

Triage: working on this.

[huonw]

(This ended up being more subtle than I thought, and will likely take a bit of rearranging of the format!() code to do properly; I'm still working on it, but slowly, so if someone wants to take over, feel free to consult with me (or not, and just open a PR, that's OK too).)

[steveklabnik]

Triage: nothing has changed here.

[steveklabnik]

Triage: no change

[sfackler]

Seems like this is something that should be over on rfcs?

[steveklabnik]

Yeah, this would be a pretty big change, probably rfc-worthy.