Add external functions (both import and export), make includes work

still need to fix literal arrays, it has the same problem as struct
literals had with moving the literal into memory and modifying the
memory with variables

test.mcl

@@ -1,6 +1,6 @@
 type str = [u8];
 
-
+include "std.mcl";
 
 struct Foo {
     a: usize,