freestanding improvements (exit, function checks)

Fixed exit for freestanding (as assert now uses it).
Running each function check now in a forked process so they can be killed or return other exit codes (and so each function runs, even if others crash)

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

@@ -1,16 +1,17 @@
 module main
-
-__global buffer [128]byte
+import forkedtest
 
 fn check_string_eq () {
-	println ("checking string_eq")
 	assert "monkey" != "rat"
 	some_animal := "a bird"
 	assert some_animal == "a bird"
-	println ("string_eq passed")
 }
 
 fn check_i64_tos() {
+	buffer, e := mm_alloc(128)
+	assert e == .enoerror
+	assert !isnil(buffer)
+
 	s0 := i64_tos(buffer, 70, 140, 10)
 	assert s0 == "140"
 
@@ -22,11 +23,15 @@ fn check_i64_tos() {
 
 	s3 := i64_tos(buffer, 70, -160000, 10)
 	assert s3 == "-160000"
+
+	assert mm_free(buffer) == .enoerror
 }
 
 fn main () {
-	check_string_eq ()
-	check_i64_tos()
+	mut fails := 0
+	fails += forkedtest.normal_run(check_string_eq, "check_string_eq")
+	fails += forkedtest.normal_run(check_i64_tos, "check_i64_tos")
+	assert fails == 0
 	sys_exit(0)
 }