builtin: add string.is_pure_ascii()

[yuyi98]

This PR add string.is_pure_ascii() and modify all the related calls.

fn test_len_utf8() {
	assert 'Vlang'.len_utf8() == 5
	assert 'María'.len_utf8() == 5
	assert '姓名'.len_utf8() == 2
	assert 'Слово'.len_utf8() == 5
	assert 'Λέξη'.len_utf8() == 4
}

fn test_is_pure_ascii() {
	assert 'Vlang'.is_pure_ascii()
	assert !'María'.is_pure_ascii()
	assert !'姓名'.is_pure_ascii()
	assert !'Слово'.is_pure_ascii()
	assert !'Λέξη'.is_pure_ascii()
}

_{Huly®: V_0.6-21194}

[Delta456]

Can you add tests for this?

[spytheman]

Excellent work.