vlib: add a new module builtin.wchar, to ease dealing with C APIs that accept wchar_t* (#18794)

vlib/builtin/wchar/wchar_test.v

@@ -0,0 +1,36 @@
+import builtin.wchar
+
+const wide_serial_number_unix = [u16(67), 0, 76, 0, 52, 0, 54, 0, 73, 0, 49, 0, 65, 0, 48, 0, 48,
+	0, 54, 0, 52, 0, 57, 0, 0, 0, 0]
+
+const wide_serial_number_windows = wide_serial_number_unix.map(u8(it))
+
+const swide_serial_number = 'CL46I1A00649'
+
+fn test_from_to_rune() {
+	for r in swide_serial_number.runes() {
+		c := wchar.from_rune(r)
+		assert c.to_rune() == r
+	}
+	assert wchar.from_rune(0).to_rune() == 0
+}
+
+fn test_to_string() {
+	mut p := voidptr(wide_serial_number_unix.data)
+	$if windows {
+		p = wide_serial_number_windows.data
+	}
+	assert unsafe { wchar.length_in_characters(p) } == swide_serial_number.len
+	s := unsafe { wchar.to_string(p) }
+	dump(s)
+	assert s == swide_serial_number
+}
+
+fn test_from_string() {
+	x := wchar.from_string(swide_serial_number)
+	assert unsafe { x[0] } == wchar.from_rune(`C`)
+	assert unsafe { x[1] } == wchar.from_rune(`L`)
+	assert unsafe { x[2] } == wchar.from_rune(`4`)
+	assert unsafe { x[11] } == wchar.from_rune(`9`)
+	assert unsafe { x[12] } == wchar.zero
+}