Better implementation of Num::add_assign.

[alex-ozdemir]

The original implementation of Num::add_assign had a few inefficiencies:

• It instantiated the standard hashmap with small keys.
• It put a bunch of items into an empty hashmap, without setting the
  intial hashmap capacity.
• It used multiple hash lookups in an insert-or-modify pattern.

This commit:

• Uses the Fnv hash function instead of the default: SipHash.
• Initializes the hashmap with appropriate capacity.
• Uses the entry API.

In my microbenchmarks, this sped up Poseidon evaluation by 1.7x.