field: Static-assert that int args affecting magnitude are constant #1345

See #1001.

Try to revert the lines in tests.c to see the error message in action.

ACK be8ff3a02aeff87c60d49883a1b2afa8b2999bbe. Verified by introducing some non-constant expressions and seeing compilation fail.

ACK be8ff3a02aeff87c60d49883a1b2afa8b2999bbe

Tested with clang 13.0.0 using the following (non C-90 conform) patch:

diff --git a/src/group_impl.h b/src/group_impl.h
index 44d9843..31e1820 100644
--- a/src/group_impl.h
+++ b/src/group_impl.h
@@ -140,7 +140,8 @@ static void secp256k1_ge_neg(secp256k1_ge *r, const secp256k1_ge *a) {
     secp256k1_ge_verify(a);
     *r = *a;
     secp256k1_fe_normalize_weak(&r->y);
-    secp256k1_fe_negate(&r->y, &r->y, 1);
+    int one = 1;
+    secp256k1_fe_negate(&r->y, &r->y, one);
     secp256k1_ge_verify(r);
 }
 
@@ -372,7 +373,8 @@ static SECP256K1_INLINE void secp256k1_gej_double(secp256k1_gej *r, const secp25
     secp256k1_fe_mul(&r->z, &a->z, &a->y); /* Z3 = Y1*Z1 (1) */
     secp256k1_fe_sqr(&s, &a->y);           /* S = Y1^2 (1) */
     secp256k1_fe_sqr(&l, &a->x);           /* L = X1^2 (1) */
-    secp256k1_fe_mul_int(&l, 3);           /* L = 3*X1^2 (3) */
+    int three = 3;
+    secp256k1_fe_mul_int(&l, three);       /* L = 3*X1^2 (3) */
     secp256k1_fe_half(&l);                 /* L = 3/2*X1^2 (2) */
     secp256k1_fe_negate(&t, &s, 1);        /* T = -S (2) */
     secp256k1_fe_mul(&t, &t, &a->x);       /* T = -X1*S (1) */

The compilation error messages are straight to the point (even without the macro expansions shown after):

src/group_impl.h:144:39: error: expression is not an integer constant expression
    secp256k1_fe_negate(&r->y, &r->y, one);
                                      ^~~
......
......
src/group_impl.h:377:30: error: expression is not an integer constant expression
    secp256k1_fe_mul_int(&l, three);       /* L = 3*X1^2 (3) */
                             ^~~~~
.....
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