Easier to use ge_madd() and ge_msub()

Easier to use ge_madd() and ge_msub()

There are two main changes:

1. ge_madd() and ge_msub() now take a ge_precomp as second argument.
2. ge_msub() can now process secrets.

The pre-computed table have been adjusted accordingly. They're now
arrays of ge_precomp instead of being multiple arrays of fe.

We can also expect a (mostly negligible) performance increase:

- The new tables have slightly better locality.
- ge_msub() is the mirror of ge_madd() instead of using it.
- Using ge_msub() for signatures is now slightly more direct.