feat: improve poseidon API

[varun-doshi]

Pull Request type

What is the current behavior?

StarkHash trait does have hash_single funciton

Resolves: #107

What is the new behavior?

• StarkHash now implements hash_single function

Does this introduce a breaking change?

No

Other information

This results in Pedersen needing to implement hash_single as well. Currently, it is set as todo!

[tdelabro]

@varun-doshi need any help?

[varun-doshi]

@varun-doshi need any help?

I'm not sure how to go about hash_single for Pedersen...since it's now in the trait for both Poseidon and Pedersen hence it needs to be implemented as well.

I can open this PR for review if that'll help?

[tdelabro]

@varun-doshi I think the pedersen hash_single impl should be pedersen(0, value)

[tdelabro]

@varun-doshi let's go with pedersen(value, 0) (based on @liorgold2 feedbacks) and make it very explicit in the documentation that there is room for collision as:
pedersen(value) and pedersen(value, 0) will return the same values
and
pedersen_array([value]) and pedersen(pedersen(0, value), 1) will too

Add the first example to the hash_single doc and the second one to the hash_array doc