Tuples and tuple indexing

[zygoloid]

Add support for extracting elements of a tuple by their numerical index.

Also formally add the well-established basic syntactic and semantic rules for
tuples, for which we have had leads issues but no proposal, into the design.

[chandlerc]

Super excited to see this in a concrete proposal!

Some quick initial thoughts inline.

[jonmeow]

To be sure:

let A: i32 = 1;
let Foo: auto = (41, 42);
// Invalid:
var b: i32 = Foo.A;
// Valid:
var c: i32 = Foo.(A);

Note, I'm not sure how I feel about this; the (1,).0 syntax will probably take some getting used to, it looks a lot like other syntaxes [and I'm super-familiar with Python syntax]. Just want to be sure my interpretation is correct.

[zygoloid]

To be sure:

let A: i32 = 1;
let Foo: auto = (41, 42);
// Invalid:
var b: i32 = Foo.A;
// Valid:
var c: i32 = Foo.(A);

You would need template A:! i32 = 1;, because we need a template constant as the index, but otherwise yes.

[chandlerc]

Approving for leads based on our discussion and merging!

[geoffromer]

Presumably this only applies to the . form, and the expression in the -> form is required to be of type pointer to tuple?

There's a similar problem on line 878.