Parser struct approach to error reporting

[dtolnay]

This is based on Sergio's work in SergioBenitez@b49a4b4. Custom error messages like the following:

error: expected 2 element(s), got 3
 --> src/main.rs:7:20
  |
7 |     demo!((a, b) = (1, 2, 3));
  |                    ^^^^^^^^^
  |
note: because of this
 --> src/main.rs:7:11
  |
7 |     demo!((a, b) = (1, 2, 3));
  |           ^^^^^^

error: expected 1 element(s), got 3
 --> src/main.rs:9:17
  |
9 |     demo!((c) = (1, 2, 3));
  |                 ^^^^^^^^^
  |
note: because of this
 --> src/main.rs:9:11
  |
9 |     demo!((c) = (1, 2, 3));
  |           ^^^

error: failed to parse: expected `=`
  --> src/main.rs:10:15
   |
10 |     demo!((c) ? (1, 2, 3));
   |               ^

error: failed to parse: expected tuple
  --> src/main.rs:11:11
   |
11 |     demo!(c = (1, 2, 3));
   |           ^

error: trailing characters; expected eof
  --> src/main.rs:12:33
   |
12 |     demo!((a, b, c) = (1, 2, 3) hi);
   |                                 ^^

Relevant to #47.

[dtolnay]

As one commit for convenient cherry-picking: 03d6727.