builtin: improve support for large arrays ([]int{len: 1_000_000_000} now works), fix an arr.repeat() bug (#14294)

vlib/v/tests/big_array_allocation_test.v

@@ -0,0 +1,19 @@
+[direct_array_access]
+fn test_big_int_array() {
+	dump(sizeof(isize))
+	mut maxn := 500_000_000 // try allocating ~2GB worth of integers on 32bit platforms
+	if sizeof(isize) > 4 {
+		maxn = 1_000_000_000 // 1 billion integers, when each is 4 bytes => require ~4GB
+	}
+	dump(maxn)
+	mut data := []int{len: maxn}
+
+	// ensure that all of the elements are written at least once, to prevent the OS from cheating:
+	for i in 0 .. maxn {
+		data[i] = i
+	}
+	assert data[0] == 0
+	assert data[maxn - 1] == maxn - 1
+	dump(data#[0..10])
+	dump(data#[-10..])
+}