builtin: support a.flags.set(.noslices | .noshrink), use it in the particles example (#13818)

vlib/builtin/array_shrinkage_test.v

@@ -0,0 +1,57 @@
+fn show_array(name string, a []int) {
+	eprintln('${name:10} .flags: ${a.flags:34} | .cap: ${a.cap:2} | .len: ${a.len:2} | .data: $a.data | $a')
+}
+
+fn trace_delete_elements(name string, mut a []int) int {
+	a.delete_many(5, 3)
+	show_array(name, a)
+	a << 55
+	show_array(name, a)
+	a << 66
+	show_array(name, a)
+	a << 77
+	res := a.cap
+	eprintln('                                        <<   ${name:10} .cap: $a.cap >>')
+	show_array(name, a)
+	a << 88
+	show_array(name, a)
+	a << 99
+	show_array(name, a)
+	eprintln('-------------------------------')
+	return res
+}
+
+fn test_array_cap_shrinkage_after_deletion() {
+	mut a := [0]
+	mut middle_cap := 0
+
+	a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+	middle_cap = trace_delete_elements('normal', mut a)
+	assert middle_cap == 14
+	assert a.len == 12
+	assert a.cap == 14
+
+	a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+	unsafe { a.flags.set(.noslices) }
+	middle_cap = dump(trace_delete_elements('noslices', mut a))
+	assert middle_cap == 14
+	assert a.len == 12
+	assert a.cap == 14
+
+	a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+	unsafe { a.flags.set(.noshrink) }
+	middle_cap = dump(trace_delete_elements('noshrink', mut a))
+	assert middle_cap == 14
+	assert a.len == 12
+	assert a.cap == 14
+
+	// Note: when *both* flags are set, the memory block for the array
+	// should NOT shrink on deleting array elements, thus << after the
+	// deletion, will still have space (till .cap is reached).
+	a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+	unsafe { a.flags.set(.noslices | .noshrink) }
+	middle_cap = dump(trace_delete_elements('both', mut a))
+	assert middle_cap == 10
+	assert a.len == 12
+	assert a.cap == 20
+}