util/slicesx: add HasPrefix, HasSuffix, CutPrefix, and CutSuffix functions

The standard library includes these for strings and byte slices,
but it lacks similar functions for generic slices of comparable types.
Although they are not as commonly used, these functions are useful
in scenarios such as working with field index sequences (i.e., []int)
via reflection.

Updates #12687

Signed-off-by: Nick Khyl <nickk@tailscale.com>

util/slicesx/slicesx_test.go

@@ -151,3 +151,49 @@ func TestAppendMatching(t *testing.T) {
 		t.Errorf("got %v; want %v", v, wantOrigMem)
 	}
 }
+
+func TestCutPrefix(t *testing.T) {
+	tests := []struct {
+		name      string
+		s, prefix []int
+		after     []int
+		found     bool
+	}{
+		{"has-prefix", []int{1, 2, 3}, []int{1}, []int{2, 3}, true},
+		{"exact-prefix", []int{1, 2, 3}, []int{1, 2, 3}, []int{}, true},
+		{"blank-prefix", []int{1, 2, 3}, []int{}, []int{1, 2, 3}, true},
+		{"no-prefix", []int{1, 2, 3}, []int{42}, []int{1, 2, 3}, false},
+		{"blank-slice", []int{}, []int{42}, []int{}, false},
+		{"blank-all", []int{}, []int{}, []int{}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if after, found := CutPrefix(tt.s, tt.prefix); !slices.Equal(after, tt.after) || found != tt.found {
+				t.Errorf("CutPrefix(%v, %v) = %v, %v; want %v, %v", tt.s, tt.prefix, after, found, tt.after, tt.found)
+			}
+		})
+	}
+}
+
+func TestCutSuffix(t *testing.T) {
+	tests := []struct {
+		name      string
+		s, suffix []int
+		before    []int
+		found     bool
+	}{
+		{"has-suffix", []int{1, 2, 3}, []int{3}, []int{1, 2}, true},
+		{"exact-suffix", []int{1, 2, 3}, []int{1, 2, 3}, []int{}, true},
+		{"blank-suffix", []int{1, 2, 3}, []int{}, []int{1, 2, 3}, true},
+		{"no-suffix", []int{1, 2, 3}, []int{42}, []int{1, 2, 3}, false},
+		{"blank-slice", []int{}, []int{42}, []int{}, false},
+		{"blank-all", []int{}, []int{}, []int{}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if before, found := CutSuffix(tt.s, tt.suffix); !slices.Equal(before, tt.before) || found != tt.found {
+				t.Errorf("CutSuffix(%v, %v) = %v, %v; want %v, %v", tt.s, tt.suffix, before, found, tt.before, tt.found)
+			}
+		})
+	}
+}