Clarify BigInt vs BigUint

[dignifiedquire]

The spec currently contains many references to BigInt, even though in most cases what is actually needed is an unsigned integer, of arbitrary size. This is mostly due to the fact that go stdlib does not differentiate, but leb-128 and cbor can both properly distinguish these, so should the spec.

[dignifiedquire]

Solved in master with the move to ipld schema