Use libtommath API for counting bits

src/big_int.cpp

@@ -296,8 +296,7 @@ gb_internal void big_int_from_string(BigInt *dst, String const &s, bool *success
 
 
 gb_internal bool big_int_can_be_represented_in_64_bits(BigInt const *x) {
-	int bits_used = (x->used-1) * MP_DIGIT_BIT;
-	return bits_used <= 64;
+	return mp_count_bits(x) <= 64;
 }
 
 gb_internal u64 big_int_to_u64(BigInt const *x) {