int128: Tidy #includes of int128.h and int128_impl.h

After this commit, int128.h and int128_impl.h are included as follows:
 - .c files which use int128 include int128_impl.h (after util.h)
 - .h files which use int128 include int128.h (after util.h)

This list is exhaustive. util.h needs to included first because it sets
up necessary #defines.

src/field_5x52_int128_impl.h

@@ -9,6 +9,8 @@
 
 #include <stdint.h>
 
+#include "int128.h"
+
 #ifdef VERIFY
 #define VERIFY_BITS(x, n) VERIFY_CHECK(((x) >> (n)) == 0)
 #define VERIFY_BITS_128(x, n) VERIFY_CHECK(secp256k1_u128_check_bits((x), (n)))

src/int128.h

@@ -1,19 +1,16 @@
 #ifndef SECP256K1_INT128_H
 #define SECP256K1_INT128_H
 
-#if defined HAVE_CONFIG_H
-#include "libsecp256k1-config.h"
-#endif
-
 #include "util.h"
 
-#if defined(SECP256K1_INT128_NATIVE)
-#include "int128_native.h"
-#elif defined(SECP256K1_INT128_STRUCT)
-#include "int128_struct.h"
-#else
-#error "Please select int128 implementation"
-#endif
+#if defined(SECP256K1_WIDEMUL_INT128)
+#  if defined(SECP256K1_INT128_NATIVE)
+#    include "int128_native.h"
+#  elif defined(SECP256K1_INT128_STRUCT)
+#    include "int128_struct.h"
+#  else
+#    error "Please select int128 implementation"
+#  endif
 
 /* Multiply two unsigned 64-bit values a and b and write the result to r. */
 static SECP256K1_INLINE void secp256k1_u128_mul(secp256k1_uint128 *r, uint64_t a, uint64_t b);
@@ -78,3 +75,5 @@ static SECP256K1_INLINE int secp256k1_i128_eq_var(const secp256k1_int128 *a, con
 static SECP256K1_INLINE int secp256k1_i128_check_pow2(const secp256k1_int128 *r, unsigned int n);
 
 #endif
+
+#endif

src/int128_impl.h

@@ -1,19 +1,18 @@
 #ifndef SECP256K1_INT128_IMPL_H
 #define SECP256K1_INT128_IMPL_H
 
-#include "int128.h"
 #include "util.h"
 
-#if defined HAVE_CONFIG_H
-#include "libsecp256k1-config.h"
-#endif
+#include "int128.h"
 
-#if defined(SECP256K1_INT128_NATIVE)
-#include "int128_native_impl.h"
-#elif defined(SECP256K1_INT128_STRUCT)
-#include "int128_struct_impl.h"
-#else
-#error "Please select int128 implementation"
+#if defined(SECP256K1_WIDEMUL_INT128)
+#  if defined(SECP256K1_INT128_NATIVE)
+#    include "int128_native_impl.h"
+#  elif defined(SECP256K1_INT128_STRUCT)
+#    include "int128_struct_impl.h"
+#  else
+#    error "Please select int128 implementation"
+#  endif
 #endif
 
 #endif

src/modinv64_impl.h

@@ -7,10 +7,9 @@
 #ifndef SECP256K1_MODINV64_IMPL_H
 #define SECP256K1_MODINV64_IMPL_H
 
+#include "int128.h"
 #include "modinv64.h"
 
-#include "int128_impl.h"
-
 /* This file implements modular inversion based on the paper "Fast constant-time gcd computation and
  * modular inversion" by Daniel J. Bernstein and Bo-Yin Yang.
  *

src/precompute_ecmult.c

@@ -14,10 +14,13 @@
 #endif
 
 #include "../include/secp256k1.h"
+
 #include "assumptions.h"
 #include "util.h"
+
 #include "field_impl.h"
 #include "group_impl.h"
+#include "int128_impl.h"
 #include "ecmult.h"
 #include "ecmult_compute_table_impl.h"
 

src/precompute_ecmult_gen.c

@@ -8,9 +8,12 @@
 #include <stdio.h>
 
 #include "../include/secp256k1.h"
+
 #include "assumptions.h"
 #include "util.h"
+
 #include "group.h"
+#include "int128_impl.h"
 #include "ecmult_gen.h"
 #include "ecmult_gen_compute_table_impl.h"
 

src/scalar_4x64_impl.h

@@ -7,6 +7,7 @@
 #ifndef SECP256K1_SCALAR_REPR_IMPL_H
 #define SECP256K1_SCALAR_REPR_IMPL_H
 
+#include "int128.h"
 #include "modinv64_impl.h"
 
 /* Limbs of the secp256k1 order. */

src/secp256k1.c

@@ -22,6 +22,7 @@
 
 #include "assumptions.h"
 #include "util.h"
+
 #include "field_impl.h"
 #include "scalar_impl.h"
 #include "group_impl.h"
@@ -31,6 +32,7 @@
 #include "ecdsa_impl.h"
 #include "eckey_impl.h"
 #include "hash_impl.h"
+#include "int128_impl.h"
 #include "scratch_impl.h"
 #include "selftest.h"