builtin: add a non allocating rune.length_in_bytes() method and tests for it

vlib/builtin/rune_test.v

@@ -11,3 +11,27 @@ fn test_repeat() {
 	assert r1.repeat(0) == ''
 	assert r2.repeat(0) == ''
 }
+
+fn test_length_in_bytes() {
+	assert rune(0x0).length_in_bytes() == 1
+	assert `A`.length_in_bytes() == 1 // latin letter
+	assert rune(0x7F).length_in_bytes() == 1
+	//
+	assert rune(0x80).length_in_bytes() == 2
+	assert `Д`.length_in_bytes() == 2 // cyrillic letter
+	assert rune(0x7FF).length_in_bytes() == 2
+	//
+	assert rune(0x800).length_in_bytes() == 3
+	assert `喂`.length_in_bytes() == 3 // hey
+	assert rune(0xFFFF).length_in_bytes() == 3
+	//
+	assert rune(0xD800).length_in_bytes() == -1 // min for surrogates
+	assert rune(0xD866).length_in_bytes() == -1 // invalid
+	assert rune(0xDFFF).length_in_bytes() == -1 // max for surrogates
+	//
+	assert rune(0x100000).length_in_bytes() == 4
+	assert rune(0x10FFD7).length_in_bytes() == 4 // "Supplementary Private Use Area-B" ¯\_(ツ)_/¯
+	assert rune(0x10FFFF).length_in_bytes() == 4
+	//
+	assert rune(0x110000).length_in_bytes() == -1
+}