Feat/encode uint64 slices

[Tabaie]

In the Plonk verification key, when multi-commits are implemented, we will have to store an array of indexes to commitment constraints. This PR enables that by storing slices of uint64 in the len(x), x[0], x[1], ..., x[len(x)-1] format.

[gbotrel]

This one might not be necessary; the default

binary.Write(buf, binary.BigEndian, slice)

will catch the slide, and encode it without the length. The only issue is for decoding, you need to pass an initialized slice as argument. Does the PlonK verification key know how many indexes to read?

[Tabaie]

This one might not be necessary; the default

binary.Write(buf, binary.BigEndian, slice)

will catch the slide, and encode it without the length. The only issue is for decoding, you need to pass an initialized slice as argument. Does the PlonK verification key know how many indexes to read?

No the length of the array is how we know how many levels of commitments there are.

[gbotrel]

OK for the PR, just, I believe we use binary.BigEndian as default encoding for everything else, just for consistency, can you replace binary.LittleEndian to BigEndian?