This macro is pretty weird -- it requires that a freestanding
`from_engine` method exists, which it uses to implement a `from_engine`
method within an impl block, by just calling through to the freestanding
method.
To reduce indirection, at a very small cost in increased repeated code
(we now need to add a `impl Hash {` and `}` and doccomment around each
freestanding function, we just remove this from the macro entirely.
This will allow us to implement `from_engine` for `sha256t::Hash` in a
different way than for the non-generic hash types.
To minimize the diff, we do not re-indent the freestanding
`from_engine`. We will do that in the next commit. However, the diff is
still a bit noisy because I had to replace `fn from_engine` with `pub fn
from_engine` in every case. I took the opportunity to also change the
return type from `Hash` to `Self` to make it clearer that these are
constructors.