Generate a new curve generator for nullifier

[XuyangSong]

We're using the NOTE_COMMITMENT_R_GENERATOR in nullifier computation. We'd better generate a new generator for it.
The generation function and test are in the constant.rs file.

[XuyangSong]

The NOTE_COMMITMENT_R_GENERATOR is also used in ShortScalar.
The generator of ShortScalar is the fixed value base of ValueCommitment in Zcash/Orchard. In Taiga, the value base is variable. It seems that we don't need the ShortScalar.
We can remove it if we don't need it; otherwise, generate a new generator for it.