builtin: optimise the common case of s.contains("x") add s.contains_byte(x) (#17702)

vlib/builtin/string_test.v

@@ -1,5 +1,3 @@
-import strings
-
 // Copyright (c) 2019-2022 Alexander Medvednikov. All rights reserved.
 // Use of this source code is governed by an MIT license
 // that can be found in the LICENSE file.
@@ -1213,3 +1211,35 @@ fn test_indent_width() {
 	assert '\t\tabc'.indent_width() == 2
 	assert '\t\t abc'.indent_width() == 3
 }
+
+fn test_index_u8() {
+	assert 'abcabca'.index_u8(`a`) == 0
+	assert 'abcabca'.index_u8(`b`) == 1
+	assert 'abcabca'.index_u8(`c`) == 2
+	//
+	assert 'abc'.index_u8(`d`) == -1
+	assert 'abc'.index_u8(`A`) == -1
+	assert 'abc'.index_u8(`B`) == -1
+	assert 'abc'.index_u8(`C`) == -1
+	//
+}
+
+fn test_last_index_u8() {
+	assert 'abcabca'.last_index_u8(`a`) == 6
+	assert 'abcabca'.last_index_u8(`c`) == 5
+	assert 'abcabca'.last_index_u8(`b`) == 4
+	assert 'Zabcabca'.last_index_u8(`Z`) == 0
+	//
+	assert 'abc'.index_u8(`d`) == -1
+	assert 'abc'.index_u8(`A`) == -1
+	assert 'abc'.index_u8(`B`) == -1
+	assert 'abc'.index_u8(`C`) == -1
+}
+
+fn test_contains_byte() {
+	assert 'abc abca'.contains_u8(`a`)
+	assert 'abc abca'.contains_u8(`b`)
+	assert 'abc abca'.contains_u8(`c`)
+	assert 'abc abca'.contains_u8(` `)
+	assert !'abc abca'.contains_u8(`A`)
+}