Add _Bool type

As requested in oriansj/M2libc#67.

The standard (C17) says in 6.2.5.2:
```text
An object declared as type _Bool is large enough to store the values 0 and 1
```

It could technically be an int but all major compilers have `sizeof(bool) == 1`.

cc_types.c

@@ -122,6 +122,10 @@ void initialize_types(void)
 	hold = new_primitive("char", "char*", "char**", 1, TRUE);
 	prim_types = add_primitive(hold);
 
+	/* Define _Bool */
+	hold = new_primitive("_Bool", "_Bool*", "_Bool**", 1, TRUE);
+	prim_types = add_primitive(hold);
+
 	/* Define FUNCTION */
 	hold = new_primitive("FUNCTION", "FUNCTION*", "FUNCTION**", register_size, FALSE);
 	prim_types = add_primitive(hold);

test/test.answers

@@ -287,10 +287,10 @@ e9fa237a2c4d945490563795fee3adeb394171d65a6025c952c818941d1b0ee6  test/results/t
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test/test1000/proof.answer

@@ -1 +1 @@
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+51a59b17d1858b5503cbb31fc6e3cae97d556c82d02e1978bcf276d07061fa83  test/test1000/proof