nit: old_ai name is misleading

[bennetyee]

oasis-core/secret-sharing/src/poly/univariate.rs

Line 71 in b0e6bc8

if let Some(old_ai) = self.a.get_mut(i) {

rather than old_ai maybe ai_ref or ai_mref for the mutable reference? i don't know what is a more proper rustacean name, but old_ai seems to be referring to the (old) value, and changing it via *old_ai = ai just doesn't read properly to me.