introduce and use SECP256K1_{FE,GE,GEJ}_VERIFY macros

By providing an uppercase variant of these verification functions, it is
better visible that it is test code and surrounding `#ifdef VERIFY`
blocks can be removed (if there is no other code around that could
remain in production mode), as they don't serve their purpose any more.

At some places intentional blank lines are inserted for grouping and
better readadbility.

src/field.h

@@ -351,8 +351,10 @@ static int secp256k1_fe_is_square_var(const secp256k1_fe *a);
 
 /** Check invariants on a field element (no-op unless VERIFY is enabled). */
 static void secp256k1_fe_verify(const secp256k1_fe *a);
+#define SECP256K1_FE_VERIFY(a) secp256k1_fe_verify(a)
 
 /** Check that magnitude of a is at most m (no-op unless VERIFY is enabled). */
 static void secp256k1_fe_verify_magnitude(const secp256k1_fe *a, int m);
+#define SECP256K1_FE_VERIFY_MAGNITUDE(a, m) secp256k1_fe_verify_magnitude(a, m)
 
 #endif /* SECP256K1_FIELD_H */