Tuples and Tuple Structs

[sunjay]

Extension of #39 with support for arbitrary tuples.

Tuples can have heterogeneous types.

Tuples:

let foo: (u8, u16, u32) = (1, 10000, 1000000);

Tuple struct:

struct Foo(u8);

struct Foo(u8, u16);

Accessing tuple fields will use special number syntax like Rust.

// get
foo.0;
// set
foo.0 = 2;

• Add numeric field access to grammar