Add emath::Vec2b, replacing egui_plot::AxisBools (#3543)

Thanks to `impl From<bool> for Vec2b` one can now shorten some builder
calls, like:

Previous:
```rust
 egui::ScrollArea::vertical()
        .auto_shrink([false; 2])
```

New:
```rust
 egui::ScrollArea::vertical()
        .auto_shrink(false)
```

crates/egui_demo_lib/src/demo/context_menu.rs

@@ -1,3 +1,5 @@
+use egui::Vec2b;
+
 #[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 enum Plot {
@@ -19,7 +21,7 @@ fn sigmoid(x: f64) -> f64 {
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct ContextMenus {
     plot: Plot,
-    show_axes: [bool; 2],
+    show_axes: Vec2b,
     allow_drag: bool,
     allow_zoom: bool,
     allow_scroll: bool,
@@ -33,7 +35,7 @@ impl Default for ContextMenus {
     fn default() -> Self {
         Self {
             plot: Plot::Sin,
-            show_axes: [true, true],
+            show_axes: Vec2b::TRUE,
             allow_drag: true,
             allow_zoom: true,
             allow_scroll: true,

crates/egui_demo_lib/src/demo/plot_demo.rs

@@ -4,9 +4,9 @@ use std::ops::RangeInclusive;
 use egui::*;
 
 use egui_plot::{
-    Arrows, AxisBools, AxisHints, Bar, BarChart, BoxElem, BoxPlot, BoxSpread, CoordinatesFormatter,
-    Corner, GridInput, GridMark, HLine, Legend, Line, LineStyle, MarkerShape, Plot, PlotImage,
-    PlotPoint, PlotPoints, PlotResponse, Points, Polygon, Text, VLine,
+    Arrows, AxisHints, Bar, BarChart, BoxElem, BoxPlot, BoxSpread, CoordinatesFormatter, Corner,
+    GridInput, GridMark, HLine, Legend, Line, LineStyle, MarkerShape, Plot, PlotImage, PlotPoint,
+    PlotPoints, PlotResponse, Points, Polygon, Text, VLine,
 };
 
 // ----------------------------------------------------------------------------
@@ -830,8 +830,8 @@ impl Default for Chart {
 struct ChartsDemo {
     chart: Chart,
     vertical: bool,
-    allow_zoom: AxisBools,
-    allow_drag: AxisBools,
+    allow_zoom: Vec2b,
+    allow_drag: Vec2b,
 }
 
 impl Default for ChartsDemo {

crates/egui_demo_lib/src/demo/scrolling.rs

@@ -146,7 +146,7 @@ impl ScrollAppearance {
         ui.separator();
 
         ScrollArea::vertical()
-            .auto_shrink([false; 2])
+            .auto_shrink(false)
             .scroll_bar_visibility(*visibility)
             .show(ui, |ui| {
                 ui.with_layout(
@@ -170,7 +170,7 @@ fn huge_content_lines(ui: &mut egui::Ui) {
     let text_style = TextStyle::Body;
     let row_height = ui.text_style_height(&text_style);
     let num_rows = 10_000;
-    ScrollArea::vertical().auto_shrink([false; 2]).show_rows(
+    ScrollArea::vertical().auto_shrink(false).show_rows(
         ui,
         row_height,
         num_rows,
@@ -193,7 +193,7 @@ fn huge_content_painter(ui: &mut egui::Ui) {
     let num_rows = 10_000;
 
     ScrollArea::vertical()
-        .auto_shrink([false; 2])
+        .auto_shrink(false)
         .show_viewport(ui, |ui, viewport| {
             ui.set_height(row_height * num_rows as f32);
 
@@ -292,9 +292,7 @@ impl super::View for ScrollTo {
             scroll_bottom |= ui.button("Scroll to bottom").clicked();
         });
 
-        let mut scroll_area = ScrollArea::vertical()
-            .max_height(200.0)
-            .auto_shrink([false; 2]);
+        let mut scroll_area = ScrollArea::vertical().max_height(200.0).auto_shrink(false);
         if go_to_scroll_offset {
             scroll_area = scroll_area.vertical_scroll_offset(self.offset);
         }

crates/egui_demo_lib/src/demo/text_layout.rs

@@ -124,7 +124,7 @@ impl super::View for TextLayoutDemo {
         };
 
         egui::ScrollArea::vertical()
-            .auto_shrink([false; 2])
+            .auto_shrink(false)
             .show(ui, |ui| {
                 let extra_letter_spacing = points_per_pixel * *extra_letter_spacing_pixels as f32;
                 let line_height = (*line_height_pixels != 0)

crates/egui_demo_lib/src/demo/window_options.rs

@@ -1,3 +1,5 @@
+use egui::Vec2b;
+
 #[derive(Clone, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct WindowOptions {
@@ -7,7 +9,7 @@ pub struct WindowOptions {
     collapsible: bool,
     resizable: bool,
     constrain: bool,
-    scroll2: [bool; 2],
+    scroll2: Vec2b,
     disabled_time: f64,
 
     anchored: bool,
@@ -24,7 +26,7 @@ impl Default for WindowOptions {
             collapsible: true,
             resizable: true,
             constrain: true,
-            scroll2: [true; 2],
+            scroll2: Vec2b::TRUE,
             disabled_time: f64::NEG_INFINITY,
             anchored: false,
             anchor: egui::Align2::RIGHT_TOP,