feat: add bitshiftLeft/Right to Nat (#613)

This is a bit outside of the regular two-argument operator functions,
but very useful for fast multiplication (and division) with powers of
two.

Provided, so the users don't have to import a primitive.

## Things I am not sure about

### naming!

Should we mention "shift" at all? If not, what should we say?

I think it is reasonable to use nomenclature as for `Nat*`.

### what about `Int`?

This can be considered as an infinite stream of digits too when're the
suffix (after dropping a certain number of bits) becomes all 0s _or_ all
1s.

Considering this as the (moral) two's complement representation, we
could define a similar operation on `Int` too.

However, proper bignums are using a separate sign bit in the
representation, so shifting negative numbers is a more involved
operation.