Parse tuple types and expressions

[mohammadfawaz]

Closes #72

Fairly simple change:

• Tuple types
• Tuple expressions
• Tuple indexing expressions. Error out on invalid indices such as 0xa or anything that is not a usize.

Note that t.0.0 does not currently work, but t.0 .0. Once we implement #66, we can then write (t.0).0. In the future, we can support t.0.0 which can be done in two ways:

• Special case the lexer to not parse the 0.0 as a real in the case of a tuple access (this means the lexer now has to concern itself with the context).
• Break the real 0.0 apart in the pasrer, which kinda what Rust does (see Accept tuple.0.0 as tuple indexing (take 2) rust-lang/rust#71322 after an attempt for the other method in Accept tuple.0.0 as tuple indexing rust-lang/rust#70420)

[sezna]

is a tuple a kind of immediate or literal?

[mohammadfawaz]

We could have something more complex such as:

let tuple = (
    { ... }, // code block
    if cond { .. } else { .. }, // if expressions
    foo(), // function call,
    etc.
);

I suppose adding tests for these would be quite useful.

[sezna]

approved but curious if a tuple could be a literal

[otrho]

What's an expr-atom? It can be any expression which has a tuple type, which might be a fn call, identifier or other tuple index. So it should just be expr?

[mohammadfawaz]

Yeah I will restructure this. See #99