I created a different version of G1Affine::hinted_check_add.

• G1Affine::hinted_check_add_prevent_degenerate() is the old G1Affine::hinted_check_add, additionally some asserts that ensure the given points do not satisfy t==q or t==-q. The caller must use this function if they are sure of this.
• G1Affine::hinted_check_add() is a more general version. It handles the excluded cases above. However, this results in substantially larger script size since it includes both the addition and doubling parts together. (618763 >> 275192)

To achieve this, I added Fq::equal_keep_elements() function that simply checks equality without consuming the elements.

We continue to use the version that excludes these cases, because they do not occur currently.

Edit: Newly added function is optimized from 618763 to 481633 bytes. It now merges the same operations for doubling and adding. And a TODO is added to hints in this function so that later we must add hint validity checks.

closes #302