Add Ui::columns_const()

[v0x0g]

Changes

• Adds a new function egui::Ui::columns_const(), which is the same as egui::Ui::columns() except that it uses a const parameter for the column count.
• Backed by an array [Ui; NUM_COL] instead of a Vec`, so fewer allocations
• Inner closure takes in an array reference, instead of a slice reference. This makes it possible to use pattern destructuring on the columns, as shown in the example, and makes it more ergonomic to use

Example

// ORIGINAL
ui.columns(2, |cols| {
    cols[0].label("one");
    cols[1].label("two");
});


// NEW
ui.columns_const(|[a,b]| {
    a.label("one");
    b.label("two");
});

Checks

• cargo fmt
• cargo clippy
• ./scripts/check.sh
• Docs
• Review