Configurable per-object item wrap count via `#[rustfmt::items(10)]` (for better displaying large vectors, arrays, & matrices)

Hello everyone,

It was suggested on Reddit (and pretty well received) to add a macro config option that allows specifying items per row within a vector or array, and aligns the items at the comma. This would be useful for formatting large collections of numbers, especially matrices where columns and rows are significant.

Here's the quote that started the discussion:

Yeah, exactly. I love using rustfmt, but just like unsafe, people shouldn't be scared to use rustfmt::skip when necessary.

For example, dealing with math heavy code, rustfmt will always reformat any large array declaration into one item per line, but there are plenty of situations where those arrays are actually matrices and formatting them accordingly is helpful. There's no way for rustfmt to ever know this.

The current behavior appears to be to use the full line then wrap if all items are short, or one item per line if any of the items excedes a limit: Example rustfmt output:

let v1 = vec![
    1i32, 2, 3, 4, 5, -34, 78, 5, 4, 6, 7, 8, 87, 5, 43, 45, 67, 8, 76, 543, 4, 567, 8, 7,
    6543, -45, 456, 7, -4, 2, 3, 45, 64, -9, 909, -940, 185, -9849484, 984, 9898, -598,
];

let v2 = vec![
    1798798798797i64,
    268763876278189,
    -29873267368763,
    782687289780998,
    -2976173718798,
    1987897,
];

// Adding a larger number to v1 makes a 40 item long vector (truncated here)
let v3 = vec![
    ...
    7,
    6543,
    -45,
    456,
    7,
    -4,
    2,
    3,
    45,
    64,
    -9,
    909,
    -940,
    185,
    -9849484,
    984,
    9898,
    -590909023328,
];

This change would allow the user to specify that they desire N items per row:

#[rustfmt::items(10)]
let v1 = vec![
    1i32,   2,   3,   4,   5, -34,  78,   5,
       4,   6,   7,   8,  87,   5,  43,  45,
      67,   8,  76, 543,   4, 567,   8,   7,
];

#[rustfmt::items(2)]
let v2 = vec![
    1798798798797i64,  268763876278189,
     -29873267368763,  782687289780998,
      -2976173718798,          1987897,
];

Even outside of matricies, this would be a nice option to enable rustfmt to make use of more horizontal space.

No files, tests, or entry points are named. Start by tracing rustfmt's existing vector and array wrapping behavior and its handling of formatting attributes; done would include the proposed per-object item count syntax and matching alignment behavior for the examples described.