std.equalRange: Compute lower and upper bounds simultaneously

The current implementation of `equalRange` just calls `lowerRange` and `upperRange`, but a lot of
the work done by these two functions can be shared. Specifically, each iteration gives information about whether the lower bound or the upper bound can be tightened. This leads to fewer iterations and, since there is one comparison per iteration, fewer comparisons.
Implementation adapted from [GCC](519ec1cfe9/libstdc%2B%2B-v3/include/bits/stl_algo.h (L2063)).
This sample demonstrates the difference between the current implementation and mine:

```zig
fn S(comptime T: type) type {
    return struct {
        needle: T,
        count: *usize,

        pub fn order(context: @This(), item: T) std.math.Order {
            context.count.* += 1;
            return std.math.order(item, context.needle);
        }
        pub fn orderLength(context: @This(), item: []const u8) std.math.Order {
            context.count.* += 1;
            return std.math.order(item.len, context.needle);
        }
    };
}
pub fn main() !void {
    var count: usize = 0;

    try std.testing.expectEqual(.{ 0, 0 }, equalRange(i32, &[_]i32{}, S(i32){ .needle = 0, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 0, 0 }, equalRange(i32, &[_]i32{ 2, 4, 8, 16, 32, 64 }, S(i32){ .needle = 0, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 0, 1 }, equalRange(i32, &[_]i32{ 2, 4, 8, 16, 32, 64 }, S(i32){ .needle = 2, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 2, 2 }, equalRange(i32, &[_]i32{ 2, 4, 8, 16, 32, 64 }, S(i32){ .needle = 5, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 2, 3 }, equalRange(i32, &[_]i32{ 2, 4, 8, 16, 32, 64 }, S(i32){ .needle = 8, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 5, 6 }, equalRange(i32, &[_]i32{ 2, 4, 8, 16, 32, 64 }, S(i32){ .needle = 64, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 6, 6 }, equalRange(i32, &[_]i32{ 2, 4, 8, 16, 32, 64 }, S(i32){ .needle = 100, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 2, 6 }, equalRange(i32, &[_]i32{ 2, 4, 8, 8, 8, 8, 15, 22 }, S(i32){ .needle = 8, .count = &count }, S(i32).order));
    try std.testing.expectEqual(.{ 2, 2 }, equalRange(u32, &[_]u32{ 2, 4, 8, 16, 32, 64 }, S(u32){ .needle = 5, .count = &count }, S(u32).order));
    try std.testing.expectEqual(.{ 1, 1 }, equalRange(f32, &[_]f32{ -54.2, -26.7, 0.0, 56.55, 100.1, 322.0 }, S(f32){ .needle = -33.4, .count = &count }, S(f32).order));
    try std.testing.expectEqual(.{ 3, 5 }, equalRange(
        []const u8,
        &[_][]const u8{ "Mars", "Venus", "Earth", "Saturn", "Uranus", "Mercury", "Jupiter", "Neptune" },
        S(usize){ .needle = 6, .count = &count },
        S(usize).orderLength,
    ));

    std.debug.print("Count: {}\n", .{count});
}
```
For each comparison, we bump the count. With the current implementation, we get 57 comparisons. With mine, we get 43.

With contributions from @Olvilock.
This is my second attempt at this, since I messed up the [first one](https://github.com/ziglang/zig/pull/21290).

lib/std/sort.zig

@@ -769,10 +769,38 @@ pub fn equalRange(
     context: anytype,
     comptime compareFn: fn (@TypeOf(context), T) std.math.Order,
 ) struct { usize, usize } {
-    return .{
-        lowerBound(T, items, context, compareFn),
-        upperBound(T, items, context, compareFn),
-    };
+    var low: usize = 0;
+    var high: usize = items.len;
+
+    while (low < high) {
+        const mid = low + (high - low) / 2;
+        switch (compareFn(context, items[mid])) {
+            .gt => {
+                low = mid + 1;
+            },
+            .lt => {
+                high = mid;
+            },
+            .eq => {
+                return .{
+                    low + std.sort.lowerBound(
+                        T,
+                        items[low..mid],
+                        context,
+                        compareFn,
+                    ),
+                    mid + std.sort.upperBound(
+                        T,
+                        items[mid..high],
+                        context,
+                        compareFn,
+                    ),
+                };
+            },
+        }
+    }
+
+    return .{ low, low };
 }
 
 test equalRange {
@@ -800,6 +828,7 @@ test equalRange {
     try std.testing.expectEqual(.{ 6, 6 }, equalRange(i32, &[_]i32{ 2, 4, 8, 16, 32, 64 }, @as(i32, 100), S.orderI32));
     try std.testing.expectEqual(.{ 2, 6 }, equalRange(i32, &[_]i32{ 2, 4, 8, 8, 8, 8, 15, 22 }, @as(i32, 8), S.orderI32));
     try std.testing.expectEqual(.{ 2, 2 }, equalRange(u32, &[_]u32{ 2, 4, 8, 16, 32, 64 }, @as(u32, 5), S.orderU32));
+    try std.testing.expectEqual(.{ 3, 5 }, equalRange(u32, &[_]u32{ 2, 3, 4, 5, 5 }, @as(u32, 5), S.orderU32));
     try std.testing.expectEqual(.{ 1, 1 }, equalRange(f32, &[_]f32{ -54.2, -26.7, 0.0, 56.55, 100.1, 322.0 }, @as(f32, -33.4), S.orderF32));
     try std.testing.expectEqual(.{ 3, 5 }, equalRange(
         []const u8,