std/lib: simplify String.{,r}index_of and improve speed for the index… (#907)

* std/lib: simplify String.{,r}index_of and improve speed for the index_of one

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

* lib/std/collections: add EnumMap.get_ref

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

---------

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

test/unit/stdlib/core/string.c3

@@ -80,6 +80,7 @@ fn void! test_index_of()
 	String test = "hello world hello";
 	assert(test.index_of("o")! == 4);
 	assert(test.index_of("ll")! == 2);
+	assert(test.index_of(" hello")! == 11);
 	assert(@catchof(test.index_of("wi")));
 }
 
@@ -90,6 +91,7 @@ fn void! test_rindex_of()
 	assert(test.rindex_of("ll")! == 14);
 	assert(test.rindex_of("he")! == 12);
 	assert(test.rindex_of("world")! == 6);
+	assert(test.rindex_of("hello ")! == 0);
 	assert(@catchof(test.rindex_of("wi")));
 }
 
@@ -98,6 +100,7 @@ fn void! test_index_of_char()
 	String test = "hello world hello";
 	assert(test.index_of_char('o')! == 4);
 	assert(test.index_of_char('l')! == 2);
+	assert(test.index_of_char('h')! == 0);
 	assert(@catchof(test.index_of_char('x')));
 }
 
@@ -106,5 +109,6 @@ fn void! test_rindex_of_char()
 	String test = "hello world hello";
 	assert(test.rindex_of_char('o')! == 16);
 	assert(test.rindex_of_char('l')! == 15);
+	assert(test.rindex_of_char('h')! == 12);
 	assert(@catchof(test.index_of_char('x')));
 }