Since these types were written, we've gained an executable spec:

https://github.com/ethereum/execution-specs

This PR aligns some of the types we use with this spec to simplify
comparisons and cross-referencing.

Using a `distinct` type is a tradeoff between nim ergonomics, type
safety and the ability to work around nim quirks and stdlib weaknesses.

In particular, it allows us to overload common functions such as `hash`
with correct and performant versions as well as maintain control over
string conversions etc at the cost of a little bit of ceremony when
instantiating them.

Apart from distinct byte types, `Hash32`, is introduced in lieu of the
existing `Hash256`, again aligning this commonly used type with the spec
which picks bytes rather than bits in the name.