freestanding: malloc/free with mm_alloc an mm_free

Added more array support that depends on malloc. Added string clone (that uses malloc). Added test for it. Eliminated stack allocated buffers from most of the unit checks.

vlib/builtin/bare/.checks/string/string.v

@@ -8,9 +8,8 @@ fn check_string_eq () {
 }
 
 fn check_i64_tos() {
-	buffer, e := mm_alloc(128)
-	assert e == .enoerror
-	assert !isnil(buffer)
+	buffer0 := [byte(0)].repeat(128)
+	buffer := byteptr(buffer0.data)
 
 	s0 := i64_tos(buffer, 70, 140, 10)
 	assert s0 == "140"
@@ -23,14 +22,29 @@ fn check_i64_tos() {
 
 	s3 := i64_tos(buffer, 70, -160000, 10)
 	assert s3 == "-160000"
+}
 
-	assert mm_free(buffer) == .enoerror
+fn check_i64_str() {
+	assert "141" == i64_str(141, 10)
+	assert "-161" == i64_str(-161, 10)
+	assert "10002" == i64_str(65538, 16)
+	assert "-160001" == i64_str(-160001, 10)
+}
+
+fn check_str_clone() {
+	a := i64_str(1234,10)
+	b := a.clone()
+	assert a == b
+	c := i64_str(-6789,10).clone()
+	assert c == "-6789"
 }
 
 fn main () {
 	mut fails := 0
 	fails += forkedtest.normal_run(check_string_eq, "check_string_eq")
 	fails += forkedtest.normal_run(check_i64_tos, "check_i64_tos")
+	fails += forkedtest.normal_run(check_i64_str, "check_i64_str")
+	fails += forkedtest.normal_run(check_str_clone, "check_str_clone")
 	assert fails == 0
 	sys_exit(0)
 }