Add functions for manipulating b256 values to the stdlib.

[otrho]

b256 values are typically addresses or other hashes which require support from the standard library.

[otrho]

Is #33 still a thing? Do we want to handle address types in the stdlib too?

[otrho]

As per a discussion on Slack, we should implement (at least) the following traits for b256:

• Ord (includes eq()).
• Hash.

Hash isn't a trait yet though, but it should be a simple wrapper around the K256 and S256 instructions for single types.

Ideally we want to be able to hash a collection of b256s, in particular a tuple pair (see #156) or array.

[nfurfaro]

@otrho Can this be closed?

[otrho]

I guess? We can/do have other specific issues for any further work.