Better embedding API support (and tests!) for strings with null bytes.

- Test that a foreign method can return strings.
- Test that a foreign method can return a string with null bytes.
- Test wrenCall().
- Allow passing NULL for "v" to wrenCall().
- Allow "a" for passing an explicit length byte array to wrenCall().

test/api/call.h

@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn callBindMethod(const char* signature);