Move _preallocated functions to separate header

src/secp256k1.c

@@ -5,6 +5,7 @@
  **********************************************************************/
 
 #include "include/secp256k1.h"
+#include "include/secp256k1_preallocated.h"
 
 #include "util.h"
 #include "num_impl.h"

src/tests.c

@@ -16,6 +16,7 @@
 
 #include "secp256k1.c"
 #include "include/secp256k1.h"
+#include "include/secp256k1_preallocated.h"
 #include "testrand_impl.h"
 
 #ifdef ENABLE_OPENSSL_TESTS