Add _fe_half and use in _gej_add_ge

- Trades 1 _half for 3 _mul_int and 2 _normalize_weak
- Updated formula and comments in _gej_add_ge
- Added internal benchmark for _fe_half

src/field.h

@@ -130,4 +130,9 @@ static void secp256k1_fe_storage_cmov(secp256k1_fe_storage *r, const secp256k1_f
 /** If flag is true, set *r equal to *a; otherwise leave it. Constant-time.  Both *r and *a must be initialized.*/
 static void secp256k1_fe_cmov(secp256k1_fe *r, const secp256k1_fe *a, int flag);
 
+/** Halves the value of a field element modulo the field prime. Constant-time.
+ *  For an input magnitude 'm', the output magnitude is set to 'floor(m/2) + 1'.
+ *  The output is not guaranteed to be normalized, regardless of the input. */
+static void secp256k1_fe_half(secp256k1_fe *r);
+
 #endif /* SECP256K1_FIELD_H */