[feat] add bits_to_num and limbs_to_num

[shuklaayush]

Adds a couple of functions to reconstruct field elements from their bit/limb representations

• bits_to_num to GateInstructions
• limbs_to_num to RangeInstructions

Assumes the bit/limb representation is valid and doesn't range check the bits/limbs.

One thing to note is that there can be multiple bit/limb representations that map to the same field element. For eg. with 254 bits and working on BN254, both the bit representations of x and x + p (254 bits) will map to x. It might make sense to range check the input to be less than p in such a case

[jonathanpwang]

LGTM